The Rule of Three is a wizened githzerai who hangs out in the Black Sails tavern, a dive bar in Sigil. He always speaks in a cryptic manner, giving three answers for every question.
Helga obtained a Merchants' Pass for Zelatar from him (27)
He passed along a message from Graz'zt regarding an assassination of the Ambassador of an Undead Drow Goddess, in return for the removal of the bounty on Skippy's head (Donat Sanp?)(31)