When 3 people are involved in 1 relationship. Sort of like a " love triangle."
Noah: "Did you hear about the freaky trilationship that Penelope, Ana and Steve have?"
Collin: " No, tell me about it?"
Noah: "Well, Ana dated Steve, but they broke up. Then Penelope dated Steve, but Steve still loved Ana, so Steve asked Ana if she wanted to date the both of them, as in Steve and Penelope. She said yes. So now, Penelope, Ana and Steve are all together."
Collin: "Sounds like a G-Rated Orgy. Fo sho."