The Dark Triad is a group of three secret organizations: the Cult of Beshaba, The Lolth Order, and The Naas (the Shuos Family).
The three groups have seen themselves as rivals in the past with there being some overlap in the sort of "work" the groups do. The three groups realized that they had a common enemy and couldn't continue to grow and reach their organization's goals without banding together and over throw the Dragons and Dragonborn rule.