You’re the Cream in My Coffee, by cimorene

“It’s traditional with married couples to throw out any excess bums, couch potatoes, or hangers-on before settling into the honeymoon nest,” he suggested around a mouthful of foam. 

Don was surprised by the attack, but he just moved forward with a damning, liquid roll of his hips and said with a thoughtful air, “I’ve always been one to buck tradition.”

Teen, polyamory. This OT3 is pretty much canon, imo – and this fic tells their story perfectly, right down capturing to the rhythm and whimsy of the movie.