Mage, Necro, and ???

[Troxx]

Shaman adds the most raw power I'd say. Shaman slows buffs regens and heals makes this trio viable with safety all the way to 60.

Other than shaman good choices are ench, second necro, second mage, cleric, bard, Druid. Best 3 options for 3rd wheel? Sham/ench with bard as either a distant 3rd or nowhere close depending on player skill. Cleric would be far preferable to an average or bad bard. Without a priest or bard you rely entirely on necro heals for players and pet chaining. Bard falls short here as well as bard can't heal pets. Without slows you'll stress out a cleric or Druid healer at the high end.

So really in the long run you'll want a healer. In the long run the potency of slow and castable buffs make shaman the clear winner.