I'm not sure, 3 mages are fine instant-aoe, BUT: sheeps only affect humans. Warlocks can fear and also ban/enslave elementals/demons. Also mages have a huge mana-problem, especially when bombing. Warlocks life-tap 3 times, the healer sets a groupheal, and you can go forward without waiting for drinking mages. Other things are of cause instant-fears, soulstones, more tanks(voidwalker) for special situations, more hp. Dots are also fine because they do damage while you control the tank/healer. If you don't control the mage for 5 seconds, they do nearly no damage.

My current favorite is 2 warlocks and 1 mage (for int-buffs that affects all other 4 chars, portals, 1 sheep and consumables).

I'm also thinking about the priest. A shaman would also be a nice dd while questing and a good healer in instances (and he buffs the whole group with his totems). I'm not sure which one is better, both are nice grouphealers and have good buffs for the group.