You can create up to four characters, and can recruit NPCs to fill any empty slots, up to six. So, if you create only three characters, you can recruit up to three NPCs (some of which, be advised, won't stay with you for the entire game). That would work just fine, though you have to take what you're offered, so you get a bit less flexibility. Or, are you saying you want to play the entire game with only three characters and not hire any NPCs? You could probably do it, but you'd be fighting at half power, so it'd be a lot tougher.
Of course, you could also play on a lower difficulty to compensate. With six characters on normal difficulty, the game isn't tremendously difficult, so three on easy would probably be manageable. Combat will take longer, of course.
Regardless, you should at least make sure to cover the Pick Locks skill, even if only up to a modest level (say 100-150). I'm not sure if you strictly need Pick Locks to finish the game, but if you don't invest in it, you'll be frustrated by a lot of locked gates and treasure chests. I also don't really recommend building 100% dedicated non-magical warriors - you will benefit greatly by having all your characters to learn Cast Spells to ease the healing and curing burden on your primary caster(s). Even a small investment to learn Heal, Dispel, and Cure Poison will make a big difference. Both Cast Spells and Pick Locks rely on intelligence, so don't just use it as a dump stat, or you'll be starved for mental skill points (see e.g. Bert).
NPCs are generally built to dish out damage and little else, so if you ignore them, you won't be missing any vital specialty skills. That also means, though, that you can't rely on them to pick locks for you.
