Yes, I have. I got together on two different occasions with a MF couple, when I was in my early 20’s. I met them on a dating website, actually.. they propositioned me and it was the first time I’d ever been attracted enough to both parties to want to actually try it out. I was more interested in getting busy with the girl, but it actually turned out that I was wildly sexually attracted to the guy. The girl was kind of blah, not very into it, very unsure, her response was very muted, kind of the opposite of what I like in a sexual encounter. The guy, however, was very dynamic, good-looking, highly sexual in a respectful and very arousing way, dominant streak, yadda yadda. Basically, all the right ingredients to make me want to get physical with someone.
Honestly, it was a bit awkward at times because I sensed the girl’s jealousy, plus the fact that she didn’t seem all that into it. In addition to that, I found it a bit odd and maybe a little bit sad to be the third wheel to an obviously loving couple (it made me miss being in a relationship, I felt somewhat left out, etc.). We got together twice in total, it was not a bad experience or anything and I definitely enjoyed quite a bit of both encounters, but I feel like I wasn’t asked for a third time because the girl was jealous of the sexual chemistry between me and her boyfriend.
The other experience I had was again MFF, but this time, I was part of a couple and the guy was joining us. Completely different experience and it was pretty damn hot. For one, my girlfriend and I had perfect trust in one another, we loved each other and were quite attached, very strong sexual compatibility, so adding another person wasn’t weird, no hints of anything negative because it was clear we were fine together and this was just something different. He was someone I knew pretty well, a very nice and respectful guy. There was no jealousy or negative feelings, just a lot of arousal. I enjoyed having an audience while kissing and doing things to her, and it was also really hot to have her watching me when it was me and him getting it on. Definitely something I’d consider doing again.. previously, I had never realized I possessed an exhibitionist streak! I’ve done dirty things in public places, but found it hard to focus because I couldn’t stop thinking of someone seeing us.
Now that I have those experiences under my belt, my ultimate fantasy is FFF. Maybe it’ll happen one day. :)