Play the Infamous demo–you can try to become a third in the G/V marriage or try to break them up (which may blow up in your face).
I liked the poly in Their Majesties’ Pleasure, because the MC didn’t feel like a third wheel, which I guess defeats the purpose of what you all are talking about, since it’s about actually making the MC a third wheel on a bicycle.
Night Market is one that will give just that to you, btw. Milo and Mal are already together, and the MC can third wheel into it by either trying for a poly or romancing one or the other of them. No matter what you do, they will be together and in love. MC isn’t really needed, but they can beg their way into it.