The last name doesn't have to come from being related. One of my friends and his girlfriend have the same last name; they aren't related, neither changed their name, or anything like that. They just happen to have the same last name. They joke that it works because even though they've been living together for almost 7 years now, they're never going to get married, but people think they are.
Sage, even though it's published by a small press (i.e. one you might not have heard of, but they pay royalties and have books in a few stores here and there), I have a novel that does end in a happy poly V situation. And it's purely contemporary; no sci fi or fantasy bits at all. It was published three years ago. In that novel, it started with a woman living with her boyfriend, who reconnects with a former lover and realizes they still have feelings for each other. The boyfriend encourages the woman to act on her desire for her friend, thinking she just needs to "get it out of her system" before they get married, but they all quickly realize the woman wants both men. And after a lot of negotiation, communication, and thinking, the boyfriend and woman invite the other guy to move in with them. It was published as a romance novel.