The HoMM2 Thieves' Guild has been rolled into the Tavern; there is no separate building for it. Buy one, get both. Open the Tavern and look for a button to access the Thieves' Guild information. Just like in HoMM2, the more Taverns you have, the more information that you can see in the Thieves' Guild. At 5 Taverns controlled, you'll see everything the Guild has to offer. Alternatively, as Plok says, visit a Den of Thieves to see full Guild information as well (but only while you're at that site).

There is no Captain's Quarters in HoMM3. You can place a hero in the garrison to have them act similarly, though heroes can level up while captains couldn't. A garrisoned hero doesn't count against the limit of 8 heroes on the world map.

-If there are no heroes in the town, then any creatures in the garrison will still defend the town, they just won't have a hero to lead them.

-If a hero is garrisoned and there are no visiting heroes, then the garrisoned hero will lead the creatures defending the town, and he will utilize whatever defensive advantages the town has available. No Fort means it is basically a battle in an open field. A Fort adds walls. A Citadel adds a single turret. A Castle adds two more weaker turrets and a moat (the type of moat varies according to castle type). A garrisoned hero cannot retreat from the battle (unless it is a Stronghold with the Escape Tunnel built); I believe a garrisoned hero can still surrender.

-If a hero is sitting in town without being garrisoned (meaning they are a visiting hero) and there is no garrisoned hero, then they will defend the town as if they were garrisoned. Be aware that the game will try to add any creatures that are in the town's garrison to the hero's army for the duration of the battle; if the hero loses the battle, then any remaining creatures that were in the garrison but didn't fit in the hero's army will vanish without fighting (or giving XP, I think). I don't know how the game decides which stacks should join the battle.

-If there is a garrisoned hero AND a visiting hero in the town, then the visiting hero that is attacked will have a battle with just their creatures as if they were attacked in the field (no town benefits, can retreat as normal); if the visiting hero loses, the attacker still has to attack the town (and thus the garrisoned hero) in order to capture the town.

Thanks again. Do you maybe know anything about artifacts crafting? I produced some artifact by using other artifacts in my equipment during game, not sure how it all works.

Np.

Certain artifacts will combine together with other artifacts that have a similar theme. All the same slots get used as if equipping the components individually, but you get extra effect(s) for wearing the combination. Each combination artifact requires an exact set of components to be worn at the same time in order to 'activate' the combination artifact.

For example, if you equip the Head, Arms, Torso, Loins, and Legs of the legion on the same hero in the hero's relevant slots, then they can combine into the Statute of Legion artifact. All 5 side slots are still needed to equip it, but you'll get an extra bonus while the artifact is equipped.

Here's a link to the list of combination artifacts, what each requires, and the extra effects: https://heroes.thelazy.net/index.php/Combination_artifact

I think I grasp it.
I would also like to ask about "PvP Options" what are these options do?

It seems that "Random map"choice does not allow to select a starting city & hero. (is there any work-around)?
By "PvP Options" I mean these 4 buttons next to castle images. I have no idea what each of them does.
'
"Random vs Random" selects 2 towns in 1 vs 1 game, but the game assigned me a completely different town than those that were selected.
https://i.postimg.cc/c6yFk0cf/Ho-MM3-scenario.png

You've got a mod installed? I use the vanilla GOG install (see the attached screenshot), but that might also explain why the Town View screen looked more like a window.

According to the options you've selected in your screenshot, you want to play a 1v1 on a Two Layer Small Map (I'm not sure why it has "XL" in the upper right and "S" highlighted). You can add more players by selecting higher numbers on the left, adding sides that can be "human OR computer" or adding "computer only" sides. There's a button there that looks like it deals with setting up teams as well; try clicking it and see what it brings up.

I suspect that you pick your starting town type by clicking the picture of one of the towns on the right side, instead of using one of the buttons next to the pictures. You might be able to select both town types in this 1v1 by clicking pictures. I don't have the mod, so you'll have to experiment some and watch what happens as you click things.

Alternatively, the text on those 4 choices next to the pictures of the towns are in two different colors, so perhaps those are toggles that you can turn on and off (like whether players are allowed to transfer gold to other players, which would definitely have an effect in a team game). If you turn off the "Random" options, then you can probably select Town Type and Starting Hero with the "Advanced Options" button at the bottom.

There are also probably some settings behind that "More Options" button.

Again, I don't have whatever mod you're using, so I'm just making guesses here based on the text I see in the screenshot. You may have to do some experimentation on your own.

I'm using only the "HD MOD". I wasn't aware that the MOD interferes with the mentioned window, According to description of the MOD that I found on the internet, allows to play in higher resolutions than native 800x600, allows as well adjust other graphics settings to make the game visually look better & includes Multiplayer mode (Online Lobby).

If you are curious and would like to experiment a little bit, you can download the MOD from the link below:
http://vm914332.had.yt/HoMM3_HD_Latest_setup.exe

Description of functionality of the buttons. (found this information on a HoMM fan polish site and translated using Google Translate).

Coin Flip - toss a coin. The game randomizes between the number 0 and the number 1. We use this option, for example, to determine the order in which players ban cities. First, the players decide in the chat who chooses "0" and who chooses "1" and toss a virtual coin. The player whose number has been drawn chooses first.
Random Town - when you click it, the game randomizes the city.
Random vs Random - in this case, the game draws 2 cities that the players will play (if they have previously agreed so). The winner of the auction decides who takes the first city of the two.
Gold transfer - an option for bidding. Players with a certain amount of gold to start (eg on the most frequently selected difficulty level - 160% it will be 10,000 gold), can "buy" priority city selection, thus giving up some of the starting gold for the opponent. In addition to bidding for cities, players also bid for priority color choices.

So, the options above do not interfere in any way with game mechanics. HoMM3 unfortunately doesn't include many maps for cooperative mode, so the knowledge on how to adjust basic settings (starting town, hero, benefits) would be extremely useful.

I know the HD Mod is floating about. I've heard of Wake of Gods (WoG) and Horn of the Abyss (HotA) as well. I prefer to just stick with the game as-is; there's enough depth and options there to satisfy my HoMM3 craving.

Look for maps that are labeled "Allies" when selecting maps that come with the game; those should be set to have teams.

For game setup (both random and pre-set), click the "Advanced Options" button. That should show a list of players, town types, starting hero, and starting bonus. See the attached screenshot for the vanilla setup. I imagine the HD Mod made some changes, but I did see the button itself in some of your screenshots.

Thanks. I sorted It out. I'm not certain about the ability called "Wisdom". How does It exactly work? Is It really required to unlock higher level spells or does It unlock additional or better quality spells within 4-5th level ones? Is obtaining the ability the only way to get 4-5th level spells or there are work arounds?

Without Wisdom you can only learn up to 2nd level spells. Some artifacts will let you cast spells while equipped, but you can't count on always getting them. Fire/Water/Earth/Air Magic skills will improve the effects of your spells.

Caveat to all the below: If you don't have a spell book, you can't cast spells. You can't cast even if you have spell scrolls or one of the Tomes of magic equipped. If the hero is going to be doing any adventuring at all, then get a spell book. It is only 500 gold at any mage guild.

Wisdom gives the hero the ability to learn 3rd, 4th, and 5th level spells. A hero's max spell level is 2nd without Wisdom. Basic Wisdom raises the max to 3rd, Advanced Wisdom raises it to 4th, and Expert Wisdom raises the max to 5th level spells.

If you visit a Shrine of Magic Thought (gives a single 3rd level spell) without having at least Basic Wisdom, you'll get a message about what spell they try to teach you and how your hero isn't wise enough to learn it.

If you visit a mage guild in a town, you'll only add spell levels up to your max wisdom (if the guild is high enough level to teach those spells). All the spells you can learn will be added, while the rest are not added.

If you clear a site (ex. Pyramid) or complete a seer's quest that gives a spell higher than your wisdom allows you to learn, the hero will not learn it. The opportunity to learn the spell is lost since you can't go back and complete the site/quest a second time. Having high enough wisdom is important!

The one exception is spells provided by items: if you have a spell book, then you can cast any spell that is on a spell scroll or in a Tome of magic regardless of your level of wisdom. For example, a hero without Wisdom who is equipped with a Tome of Air Magic can cast Summon Air Elemental. Note that these items don't teach spells; if you remove the item, then the spell is removed from your spell book if you haven't learned it from somewhere (once the hero knows a spell, they know it for the rest of the map). Also, the hero has to have the item(s) equipped, not just sitting in their backpack.

Thank you for clearing things up. Another thing I don't understand in this game are number of moves in battles.
For instance, in the battle shown in the picture below, I could only move one unit, then the enemy (CPU), was able to move all their units until my turn. Why is that so?

The movement order of creatures on the battlefield is determined by their speed (plua or minos any effects of artifacts of spells). The wolfrider is very fast, because of that he moves first, the thieves were next in line.

Based on the picture, it looks like your hobgoblin goes first, then your wolf rider gets a turn, then all the enemy rogues get a turn. The fight is happening on native terrain for the Hobgoblin and the Wolf Rider, so they both get +1 speed for this fight, which makes the Rider faster than the Rogues for this fight. The Hobgoblin was already the fastest unit in the pictured fight, so it goes first. The turn order listed in the combat window verifies this. Based on the creature numbers, creature types, area of map revealed in the mini-map, hero picture+stats, and the initiative tracker, I'm guessing you don't have Advanced Tactics, which means both of your creatures got their turn in Round 1. I see nothing wrong in the screenshot.

Combat is divided into rounds and by default each creature gets one turn per round. After they take their turn, high morale is rolled (if applicable) and may give them an immediate extra turn in the round (a given stack can only get high morale once per round). Low morale (if applicable) is rolled at the beginning of the creature's turn and may cause them to lose their turn entirely for that round. Disabling spells and effects (ex. Blind) may cause a creature's turn to be skipped, but if the effect is removed before the round ends, then the creature will take their turn at the appropriate time in the round.

Each round has two different halves. In the first half, creatures act from fastest to slowest according to their speed, then in the second half all creatures that used the "Wait" command in the first half will act according to their speed from slowest to fastest.

In the first half, where creatures act from fastest to slowest, the game checks for the highest speed that hasn't had a turn yet, and makes that creature the active creature. If multiple creatures on the same side have the same speed, then the tie breaker is which slot they were in when the battle started; slots closer to the top of the screen act first. If creatures on opposite sides have the same speed, then the tie breaker is lost by the side who last had a creature act. For example, if your Rider and the enemy Rogues had the same speed, then the turn order would be Hobgoblin (Highest speed) -> Top most living Rogue (tied with your Rider, but your side was the one to most recently act) -> Wolf Rider (now the Rogue team was most recent to act, so your Rider beats the next Rogue) -> Rest of the Rogues from top to bottom. If no one has had a chance to act yet (because the battle has just started) and creatures on opposing sides have the same speed, then the Attacker wins the tie (ex. if both sides have a single stack of Goblins, then the Attacker gets the first turn).

Once every creature has had a chance to act (or been skipped because of disabling effects), then the second half of the round starts, and the game starts from the lowest speed of creatures that haven't acted yet and goes up. High speed creatures are able to "Wait" longer than low speed creatures. The same tie breaker rules apply as in the first half of the round, and the game doesn't "forget" which team acted most recently at any point.

Creatures retain their "top to bottom" initiative tie breaker for the whole fight; it doesn't matter where they move during a battle, they'll still have that same priority on their speed compared to other creatures on their team.