Mages are insanely convenient in that you can make your own food/water and can teleport. If you go frost, you get a very powerful pet on cooldown, and a very powerful shield. 3 mages with Ice Barrier up can do tons of AoE damage before taking any.

Warlocks are great in that you really get 6 chars for 3, plus having a tank is nice. Another bonus from the a lock is being able to SS all 3 in case of wipes. Their AoE is a little tricker since it requires you to click for rain of fire, or take damage for hellfire. Multiple DoTs, however, will dish out tons and tons of damage while you alternate fears and howl of terror between the chars.