I'm in an open/poly relationship. Me and my fiance have had a year long crush on our friends who are a monogamous couple.

One of them is finally starting to flirt back with me and suggesting a threesome with him and his girlfriend. I'm scared I'll possibly fuck up their relationship if things go further. Plus my fiance may get jealous that I got to sleep with them if he wasn't invited...

sigh

Does the girlfriend agree 100% on the matter? because I can see it get tricky if she only agrees on it for his sake. Until you're absolutely certain that that's what she wants as well, only then should you consider it. As for your fiance, he needs to know about the development too, so he can make a rational decision whether he can live with it if only you get to sleep with them. I would say that sleeping with them or not, it's your choice, but respect for your primary partner's feelings is key here. But who knows; if the couple are both up for it, you can always ask if your fiance can join too :)

This hasn't been formally discussed between me and them, but if it ever were to come up as a legitimate possibility, I would need her to be 100% on board completely. I've discussed it heavily with my fiance, and he's fine with it. However, his feelings may be different than he expect should it actually happen.

Well, then it's probably time to arrange that talk with them and see where that goes. Nothing can really be done about your situation until that is out of the way :)

I'm much too nervous to bring it up. I just kinda have to wait until they do. They know I used to have a crush on them, pretty sure they still think I do. Which s right, but eh.