Yes. FFM. Twice.
The same (bi) GF was involved in both scenarios, but the second girl was different on both occasions.
The first one was really just a bit of strip poker, kissing, groping and fooling around which culminated in my gf giving me a blowjob while her friend watched and played with herself.
The second one was the full blown porn movie type of scenario. My gf let me hook up with another woman we both knew was bisexual, and it was arranged that she would come home while we were in bed together (the second girl knew this as well).
TBH, I am glad I did it, just for the experience, but although it was pretty amazing (it went on from 4pm on one day until 5am the next), I didn't enjoy it as much as I thought I would (perhaps all fantasies turn out like that?).
Stand out moments were
- The moment when I knew it was really going to happen (when I started fucking the second girl and my gf came over and french kissed her)
- Fucking one of the girls doggy style while they 69'd. The lower one would take my dick out of the top girl's pussy and suck it before guiding it back into her. The girl who was on top in that position also had the most amazing ass I have ever encountered in the flesh.
- Three way french kissing.
- Both me and my gf going down on the second girl at the same time.
- Both of them blowing me at the same time
Inevitably, as these things often do, the whole thing went tits up. The second girl wanted to see me outside the 3 way thing, and I was keen to do so (my relationship with my ex was on the rocks at the time anyway), but this caused the whole dynamic to break down, and it never happened again. My gf and I also split shortly after.
I would say that if you get the opportunity, then you should do it, but bear in mind that if you are doing it within a relationship, then that relationship needs to be very strong or the people need to be the non-jealous type, otherwise it can all go horribly wrong afterwards.