Ruins of the Old Empire
The Triune Evil is the collective name given to the three evil gods responsible for the Curse on the World. It’s considered unlucky to name any of Nerull, Torog, or Beltar, so the name developed as people tried to talk about them and their hatred of the Imperial races without risking their attention.
The name is also appropriate since, in the days since the fall of the Empire, the three have been worshipped in concert by evil humans rather than individually. In a sense, the Triune Evil is the flipside of the dual religion of most people, who worship Amaunator, Pelor, and one other good or neutral diety of their choice.