When two guys have sex with one girl. One rule of a devil's three way, as described in the Bro Code on How I Meet your Mother, is that guys are to never make eye contact while involved in a devil's three way.
What you claim is a drinking game when you're a Supreme Court nominee who doesn't want to admit in public that you and another guy both fucked a girl at the same time.
Senator: Judge, what does the "devil's triangle" entry refer to in your yearbook?
Judge: Sir, it's a drinking game.