I've played this game three times, the main thing is to have:
1) Paladin / Cleric (healers)
2) Sorceress (teleports, combat magic)
3) Druid (versatile mage)
4) I also took various other heroes. Even a Knight :) although the Knight frankly becomes a burden later on, sometimes he still provides salvation (lots of HP)
The most difficult place in the game is in the Darkmoor castle, where there are a lot of flying eyes. In Might and Magic 6, they are much stronger than in Heroes 3 :)