The three High Priests were high ranking clergymen in the Temple of the Triune. Each order was lead by a High Priest, who preached to the pilgrims and acolytes of his particular order. The three High Priests answered directly to the Primus.
Due to their high standing and proximity to the Triune's leadership, the three High Priests were granted many boons - most demonic in origin. They were blessed with unnaturally long lives, or mastership over the arcane arts of demonic magic.