I'd go with a paladin tank and priest healer for the duo and 3 shamans for the triple boxer. or 3 mages. both are good options.

But I like simplicity. Shamans cannot go oom with the combination of judgement of wisdom and thunderstorm. I havent been able to test it for long durations since these were all mostly in instances where the fights are maybe 3-4 minutes at most, but i almost always end fights with 100% mana >_>