I have a bad feeling about him, granted I don't know all the details but you have a chance now to end this while it may be painful now it doesn't compare to a problematic marriage and or divorce in the future. Any problems you have now will magnify by 1000x after you guys actually get married. This seems to go deeper than just the bestfriend. Btw that "bestfriend " is shit, seems like she's hungry for attention. But that besides the point , the fact that he's even mentioning that makes it seem like its still in his head. I mean theres also a small chance that this is a test and he wanted to see if it could have happened back than ( or even now), why else mention it? You sound like you tolerate a lot of shit you shouldn't be . Having a "bestfriend " who flirts with your boyfriends like crazy and than taking back this guy. You probably have a big heart and a lower than average self esteem. Anyway I think the reason you " overreacted " (not really) is because the issues go deeper than him mentioning this threesome idea, you know something you dont want to admit to yourself and your having second thoughts about the dude. This is what I could gather from the info you provided.