Ex gf was bi and after some discussion she wanted to do a three way. It was exciting. I felt like I did it. The pinnacle of sexual encounters for the average guy non pornstar! The sex was awesome, and a but refreshing getting to feel another woman. After we were done the girl left and immediately a fight broke out. Long story there she complained I put more into her friend than her. I retorted I didn't mind when their faces were buried in each other's pussys. It didn't go well but time healed it for the most part. Fast forward a year and out of nowhere she brings it up and resentment builds. We go out drinking one night and run into an old friend who me she had a crush on years before we started dating. We were drunk and she in a cutesie way says we should have a three way with him. I told her hell no. One you are bi and having another girl was just as gratifying to you as it was me. She throws a huge fit and in my drunken state felt like my god if this will make all this resentment go away fine. I layer ground rules that he couldn't fuck her only mouth stuff. She agrees and we took the guy friend home. We start messing around and right away I'm uncomfortable and can't get it up, so I pulled her aside and explained I couldn't do it. She said ok that's fine it was getting kind of weird. I felt so relieved. I went to take a piss and came back to them fucking. Not just fucking but he was straight anal fucking her. I blew up and got pissed pulled the guy off and out of her and he came all over her and threw him out. We'd been together for 5 years and I had no idea what to do after that. We tried working things out but after that she went off a deep end and ended up starting to cheat and mess around behind my back until finally breaking up.

Tldr: three some ruined my relationship. Some fantasies are best left fantasies.