n. Three people in a romantic relationship.
2013
In that case, every argument for recognizing two men's bond as marital — equality, destigmatization, extending economic benefits — would also apply to recognizing romantic triads ("throuples," as they are now known).
2012
"Tim is wide open to defining who he is sexually," says James. "The setup is that Tammy’s and my character consider a relationship with Sean. We’re calling it a throuple." ("Threelationship" also works.)
2004 (earliest)
It's the hottest thing in romantic commitment since monogrammed towels. It's set Provincetown abuzz. And now it's got a catchy name: "throuple," as in couple, only consisting of three (usually same-sex) members instead of two.
A less popular but more euphonius synonym is threelationship, which first appeared in a December 12, 2007 Urban Dictionary post.