If you’re only doing 3 classes, and it sounds like it’s more or less for PVE.
Pally tank (good idea)
Healer, I personally like shaman for this, chain heal, mana totem, Earth Shield etc.
Now for DPS choice
Lock – Easy to setup DPS macro (from experience at least) Water breathing buff is handy, CC, HP buff from pet, Summon, Off tank (if needed?)
Or
Mage – Free food/water forever. Int Buff, CC. Portals.
I’d go lock but that’s me. It’s easy to fill DPS slots rather then find a healer.