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 (Donat Sanp?)(29)