I had one with my ex-wife and her friend from college. It had no impact on our relationship and we often talked about doing it again. And to clarify, she's my ex now now not because of this, but for other reasons unrelated (serious).
Backstory: Before it happened we talked about it quite a bit. Mostly during and after sex. Initially it started out more as a fantasy but over time it became something we both wanted to try. We debated over how to find someone because we didn't want to select a complete stranger and we couldn't pick someone we knew too well or saw often. It had to be someone that we knew well enough and someone that wasn't easily accessible. That's when we started talking about her friend from college who lived out of state. She was someone that was hot (met looks and body requirements for us BOTH) and someone we knew. In fact, my ex and her would often talk about sex on the phone but it never clicked until we started looking.
After talking about this friend as a possibility, my ex started talking to her about participating in a threesome with us to hear what she thought. It was a natural extension to their usual conversations, and he wasn't pushy with it. The friend was initially cautious but we also knew she had already been in a threesome several years prior. That was both good and bad. Good because we knew she would hook up with a woman. Bad because that threesome destroyed the other relationship. As a result she was cautious about ruining our relationship and marriage.
My ex continued to talk to her about it and after while I just forgot about it because the conversations happened over the course of months. Then an opportunity popped up! She was going to be in town overnight for a brief one day visit while heading elsewhere in the states, and she needed a place to stay. In hindsight this was no coincidence because she could have opted for a hotel.
Anyway, the night she arrived my ex and I were super excited but I maintained some degree of reality. She hadn't seen me in a few years. I hadn't seen her. Would we all be attracted to each other? Would she pretend my ex had never brought up the topic? She also arrived in town late (around 7pm) so there wasn't much time.
After my ex picked her up from the airport we sat down for drinks. She was still hot and we all got along. We chatted for a bit, but the conversation wasn't going in the direction my ex and I hoped and stayed more superficial. It got later and I ended up going to bed thinking it probably wouldn't happen. I also had to work the next day. We all ended up saying good night and went to bed. My ex and I laid in bed for about 30 minutes debating what to do. We were both horny and knew we had the opportunity. After some debate, my ex texted her and asked what she thought about joining us. After an agonizing several minute delay of no response she responded with something like "maybe? I don't know?! I can probably be talked into it." I guess she was laying in bed thinking about it.
I rushed my ex out of the room to go get her but she ended up being gone for about 15 minutes. I laid in bed thinking she was trying to get her up. My stomach was in knots with anticipation. Just as I was about to investigate I heard some rustling in the hall and my ex walked through the door holding her friends hand. I later learned that her friend asked her to get into bed to cuddle first to get warmed up. They ended up having their own warm up session and lost track of time! When they joined me in the room I jumped out of bed. My ex undressed her and we all got into bed and had sex.
In the end, that event didn't change our relationship because we prepared ourselves for it and selected carefully. In fact, it made our sex more interesting and open. Can't guarantee it would happen that way again if I were to try it again with other women. Definitely would like to do it again, though.