Let’s 1st get this out of the way- I’m a veteran of tactical TBS games. It’s one of my favorite genres. I’ve completed classics like FF Tactics, Vandal Hearts, full campaigns of Panzer Corp and Allied Corp, and a slew of turned-based JRPGs...etc. I say this not to brag (a lot of people completed those games too). But the point is, I’m no slouch. And here’s the thing; HOMM3 might be the best I’ve seen yet, even after playing it for just 1 week. And maybe my inexperience with the game could be the issue, but some of the rules and game-play mechanics just don’t make any sense. Ya know what game comes to mind? - “Divinity: Original Sin” (one of my all-time favorites). From the very start, you get full control to create your two characters. It took me an hour at least.

HOMM3 starts the campaign out with one hero- Christian The Knight, no choice. No choice of difficulty settings. During the campaign, you flesh out your heroe(s) with spells and secondary skills that get thrown at you AT RANDOM...again no choices. Well, actually there is a choice - between Basic Navigation and Basic Learning...really, that’s just what my knight needed. Now, don’t get me wrong, I understand this is an old game and games have come a long way since then.
I guess I’m just used to having full control. In Allied Corp (I believe it’s Slitherine), OMG, in prep. for the next mission, I’d spend 15 minutes deciding on what I should invest my prestige points on: upgrading units, buying more tanks or artillary, no, I got it - more air support!

But back to HOMM3, I’m currently on the 3rd mission - “Griffins Cliff”. According to a couple of strategy guides, it is advised in the beginning to split the armies from all of your heroes and give them to your two best heroes. First of all, there are no armies. Hey Richie, remember all those great creatures and artifacts you had from the last mission...see ya!
WHAT? Secondly, it is then advised to head straight to the underground (with your two heroes) to take out the three towns the orange enemy hero currently has control of. In that way, the orange enemy can’t build other strong heroes. Makes sense I guess. Except for two main problems: 1) My two heroes took out the three towns, (but lost some troops). After that then what? You can’t leave them there, they’re your two best heroes. But if you leave, asshole orange comes back and takes out your new hero that you bought to guard the place...geez! 2) And my favorite bit of advice is to have the scouts that you left upstairs run around and collect resources and build your towns. Well, here we go again. It just so happens that a good portion of the resources are guarded by strong creatures...good luck with dat!

After a few ‘in-game’ weeks of this nonsense (I say nonsense, because let’s face it, I suck!) all hell breaks loose! What seems like a never ending squadron of enemy heroes go from one end of the map to the other as if they were on a flying carpet and one-by-one take out my heroes. NO, Valeska, NO. Ah shit! Screw this!

Before I sign off, let me state another observation that I made about this game that relates to my point in paragraph two.
i’ve watched three separate walkthroughs of this mission on Youtube. In all three walkthroughs the gamers had the same three spells available to them that for the life of me, I still can’t figure out why they have ‘em and I don’t -‘Chain Lightning’,
‘Meteor Storm’, and ‘Fire Ball’ - devastating spells that do area damage to groups of creatures. What?, Was I just unlucky that these spells didn’t endear their randomness to me? It seems the only 3rd and 4th level spells I have are Protection against this, Protection against that. Gimme a freakin’ break!

Sorry for the long post. I am not giving up on this game. It is my new game passion and I will conquer it (or at least win a few campaigns anyway). And I still think it’s awesome and in all my sucktitude, i still think it’s fun. But, I haven’t got a clue what i’m doing wrong. I mean, not a clue! I will have more questions in future posts for sure. More specific ones, like why the hell didn’t the mana vortex give me double mana. And what the hell is a garrison hero? Until then, expert HOMM3 experts, I need your help!

First of all you should know that 99% of all gaming videos put up on YouTube are by wankers who minmax, save scum and grind to the max.

In the HoMM 3 campaign make sure you recruit the at least as many heroes as you are allowed to carry over to the next scenario.

Your first priority should be to build up your castle. Normally, at least if you play on Impossible it will be best to concentrate on buildings that increase income, but in the unmodded campaign it's probably better to concentrate more on creature dwellings.

Send out scouts to pick up unguarded resources, and to explore. Exploring is extremely important. With no fog of war (except for the Necromancer induced one) it mean you easily keep track of enemy heroes in explored areas.

Have you main hero concentrate on liberating guarded mines and treasure troves in the beginning.

When you invade, try to conquer enemy castles on day 7 or day 1. Then you can quickly recruit a new army, and also deny the enemy the same troops that week.

Something PetrusOctaviuanus said bears repeating: recruit the number of heroes that will carry over to the next mission. Expanding on that, those heroes (with their current level, stats, and known spells) will carry over, so send each one to every boosting site on the map before you complete the campaign map (if possible; sometimes time limits might limit you). If a hero hits the level cap for a mission, then transfer their army to someone else you want to take with you to the next map and let the experience flow; you can send your level-capped hero to start doing scouting duty, or hitting the weekly resource sites, or even making a loop of the map to hit all the stat buff sites.

-If you make a point of only using heroes within a single faction (in this case, only use heroes aligned with the Castle town), then you'll be able to group all of their units on a single hero to make a larger army.

-These heroes are going to be with you (hopefully) for the rest of this campaign, so pick good ones. Don't take Sylvia if you could take pretty-much-anyone-else. If you don't like Christian, then just hire someone else and use them as one of your carry-overs.

-Make sure to build the highest Mage Guild you can in every city, and send each hero to each town to learn spells.

-Every hero you transfer should have Wisdom (preferably Expert, to learn spells from each map), Earth Magic and/or Air Magic, and give exactly one of them Scholar (to teach later heroes all those spells from earlier maps). If they can get Logistics, then that's great!

-If a hero isn't doing anything else, then have them either scouting new terrain (and collecting unguarded anything) or hitting the sites that give stuff each week.

You did RTFM, right? : )

1) Don't worry about what spells you get. Each guild and each map will be random, but as you go through multiple maps, you'll collect most of the spells, so it's more a case of when, not if, you get them. Do know that some maps may ban certain spells, and the guilds in different towns are weighted to have different spells. For example, you'll get far more fire spells in a demon town over a castle.

2) The first week is crucial. Start strong and keep pushing. You need to be aggressive.
a) On the first day, hire both heroes with units. Give their troops to your main guy leaving them with only a single fast unit, then use those two to scout and scoop. Explore what you can and collect what loose goodies you can. If there's a lot of water, send someone out on a boat asap. If given a choice, explore areas closest to you first. Explored areas will reveal enemy heroes sooner, giving you more time to defend your home or move people around.
b) Your main guy should secure wood and ore mines first to help build, then luxuries as needed. Clear paths to let your scouts explore more. Go for buildings that will boost your main hero, and paths to get to unexplored areas first. First team to an area gets the loose goods there, and you want that to be you.
c) My general rule for town building is:
gold
monster 1-4
citadel
monster 5-6
castle
monster 7

If you have gold but no troops and lose your town, you can flee and get troops elsewhere. But if you have troops and no gold, when an enemy takes your town you'll have nothing. Now they'll buy all those troops and use them agaisnt you. Gold first and foremost, gold gold gold. Get a capitol up asap.
d) Second week hire two more scouts if needed (probably) and give their troops to your main guy or leave in the town (to deter the computer)
e) Over time select the heroes you want for your permanent group, and develop them up as Bookwyrm said.

Also griffin Cliffs is last mission in this campaign, and this campaign is the easiest one. If You do not like campaign try first learn game basics on single scenarios maps, as it would later pay off when coming to campaign. And remember that You have autosave. :P Helps when You fall into some bad Witch Hut with navigation, or when You attack Horde of archangels.

"Explain to a beginner what this game is really all about."

Without having played any of the other games mentioned, I reckon games in the "Heroes of MIght and Magic" series are much more like "Panzer Corps" than any of the others, albeit dressed up in a fantasy-world setting. HoMM games belong more to the strategy genre than the tactical. Starting with a small (relative to the map) warband, you have to grab resources to get your bandwagon rolling so more warriors will join up, but remember this is just the means to the end, the end being to wipe out your rivals.

The "Might and Magic" series started first, with HoMM as an offshoot. If anything, the names are the wrong way round, because Might and Magic games are about a handful of adventurous individuals exploring, as a party, a world in which there are dungeons and dragons. One can spend time at the start setting up the characters. Whereas HoMM games are about (small-scale) warfare between the followers and hirelings of rival warlords. In the HoMM3 campaign you are on, by the way, it is Queen Catherine you are playing as (radioing instructions from the fuhrerbunker, as it were), not Christian your knight. He and the other "heroes" are sort of mercenary field officers you need to command battle groups.

Much of the mechanics you are meant to discover as you go along, by trial and error, so once you've got through a scenario expect to think you could go back, have another go and do better. One important mechanic (which some Lets Players on YouTube neglect) is that, with the mobs of creatures placed in everybody's way, there is usually a chance (if a small one) that when attacked they would rather enlist for free than go down fighting - provided there is room for them in the attacking army.