If I understand correctly, you can not get the thievery skill in the game so you have to choose either robber or ninja as one of your characters (or get them later in a tavern) in order to be able to open chests and finish some quests.
Or can you open chests without the skill (just with more tries)?
Thanks

Anyone can try to open chests, locked doors, and locked gates. But if a chest is trapped, you will set it off. And if a door or gate is locked, you'll need to (B)ash it open. Don't entirely remember if you can (B)ash a locked chest.
I use the Ninja myself, and I make sure to give the bulk of Resistances I come across to the Ninja since he's the one taking the most Elemental damage (from exploding chests, doors, and gates).
One major upside to having a Ninja/Robber in the party is that they get XP for opening chests, doors, and gates on the premise that they are using a Skill to disarm the traps & pick the locks. So the Ninja/Robber will typically be a couple of Levels ahead of the rest of the party, which helps make up for the fact that he's taking damage more often (until he gets his Thievery up there).

You can pick a few chests or doors at the beginning without thieving skill, but later it will be impossible without a thief or ninja in the party. Doors can be bashed, chests can't.
It's possible to finish the game without a thief/ninja, but you'll have to leave almost all chests unopend and won't get quest items from chests.
You won't be able to enter Necropolis (and another area) without a thief/ninja.
So take a thief or ninja (I recommend dwarf), besides stealing they can fight quite well (ninjas have the same attack power as knights) and because of the faster levelling they can use magic items very effectively.

I have already created my party wihout a robber or a ninja and I have already reached level 12.
So now I have three possibilities:
1) start over again
2) hire a robber or a ninja from a tavern and "spoil" my party
3) use a trainer and get the skill this way
Only 1 and 3 is passable as I really like creating and naming the party.
Still one thing remains in my mind - I just don't want to have a robber in my party. It is againt my ethics.
Ok, ok, starting over again and exchanging my archer for the ninja losing possibility to have another caster od sorcerer spells.
So the party says:
Barbarian
Knight
Cleric
Sorcerer
Ninja
Paladin

You could keep your current party, and just replace one character. They'd be a bit behind the rest of the group, though, so you'd have to babysit them for a little while.

Yeah I always thought it was a little weird that they were called "robbers". I prefer to think of them more like rogues, people with skills in disarming traps and picking locks that don't necessarily use them to steal. Remember that you can still choose to act nobly in the game, i.e. not steal things, but rather use those skills to aid your party by helping with traps you come across and opening chests and doors that you encounter.
And another alternative is to use a ninja instead, as you have chosen, who doesn't come with the same bad connotations as a robber.
One further suggestion: With the current party you listed, you have three characters with strong combat ability: the Knight, Barbarian, and Ninja. If you wanted to have a second caster of sorcerer spells, you could keep your archer (also a good fighter) and swap out either the Knight or Barbarian. It's been a while since I played Xeen (I'm starting from the beginning and am only just now finishing MM2) so I forget the exact difference between Knights and Barbarians, but I think Barbarians can't use all the equipment that Knights can so I might say keep the Knight in that situation. Of course, it's not necessary to do this swap, only if you wanted another caster of sorcerer spells.
Don't worry too much about the ninja being behind the party in levels... he/she will gain experience quickly since everyone else is higher. The only problem might be that you will be encountering tougher traps so the ninja might have a lower success rate at first.
Oh, one more thing... if you make the NInja in the Inn on Darkside, he/she will start at level 5, so there won't be as much of a level gap.

Barbarian is the by far most powerful class for physical combat in MM3-5.
Ninjas loose only 3 HP per level and a few unimportant weapon/armor options compared to knights (you can still use everything since you've a paladin in the party and the extra lockpicking XP will make the Ninja even more powerful than the knight), so I'd rather replace the knight than the archer with a class similar to knight.
But your team will work too, it has much fighting power and average spell power. Even with 4 non-casters, a paladin and an archer the game is beatable without too much trouble.

I would have traveled to Darkside and swapped out the Barbarian for a Robber or Ninja. In a well balanced group, a Barbarian is less useful than an Archer. I say Darkside because characters created on Darkside start at level 5, whereas those created in Clouds start at level 1.
A difference of 7 levels at the start of the game is about the same as 1.5 levels later in the game. That is, your new character starting at level 5 should reach level 11 before the rest of your characters reached 13. And from then on he would basically remain just 2 levels behind. Two levels, especially later in the game, is nothing. Especially when you consider that towards the end you're likely to have far more experience than you can actually train with.

You can't learn Thievery in Clouds/Darkside/Worlds of Xeen. Oddly enough, you can learn the skill in Might & Magic 3 as well as in the Swords of Xeen game.

If it's a robber or a ninja it might actually just catch up to the group.

It's possible to hire a thief for picking locks.
Collect items which raise thieving skills and note where the chests are which you can't unlock.
Go to the darkside inn, create a gnome robber and replace a party member. Equip all items which raise thieving skills, teleport to the well/shrine which raise levels the most and to the chests you left behind. If the thieving skill is still too low the XP from the easier chests, the throne for thieves and the book for princes should yield enough XP to gain some levels without taking XP away from the party members.
When the thieving work is done take your party member back from Castleview.
But I don't recommend playing without a robber or ninja. You won't feel more powerful, you'll feel more annoyed.

Actually, I started over again. Now all characters are level 5, only ninja is already level 6.

Thanks for this thread!
Playing MM3 in 2023. Going to beat it this time. Life goals!
I have a ninja in my team. His Thievery skills is 54,
and he can't even open chests in RedHot Blistering Heights.

Some questions...

1. Is there a way to improve Thievery skills in the game?
Any throne or magic well that would add Thievery skills?
2. Or do I have to rely on equipment that augment Thievery skills

Things that affect Thievery:
* Race. Certain races have racial bonuses or penalties that apply to the skill.
* Class: Robbers get +30, Ninjas +15. (If you somehow get the skill on someone else, you don't get this bonus.)
* Level: Each level is +2. This bonus, I think, is the only one that requires actually having the skill. In particular, note that temporary levels will also boost this.

Also, note that you don't need every single treasure. There's plenty of opportunities for Obsidian equipment, and there's one chest in particular I'm thinking of that you might want to skip. The chest in question requires huge Thievery (200+ may not be enough), and even when you successfully open it, it can do as much as 10,000 points of fire damage, probably turning some characters into grave stones.
.
Especially since Robbers need less XP than anyone else.