Technically, three people together for any purpose, but most commonly used for three people having sex. Also a fairly good movie about the relationship between one woman and two men accidentally assigned to share a college dorm suite.

a fairly effective method of dismantling the emotional aspect of a couple's relationship. used initially by a participant of a relationship under the pretense of expanding and diversifying the couple's sex life (which usually proves positive in an relationship, overall), though the true intent (concious or subconcious) or if not, final result, is to terminate the relationship indirectly.