Depending on why your limiting yourself to 3 I guess. But if your just doing a little PvP and open world PvE, i would pick druids and or rogs for stealth!

BUT! The most fun 3 man team is the one that adds two more and becomes 5 man! I have tried to do just 3 man, but then I always regret it when i level up less than 5 and end up having the play catch up!