No such luck in my case. Can't say I ever had a girlfriend who was quite that adventurous, although, I did have a friendship (and occasional FB relationship) with a woman who was pretty open. I believe I could have had either an MMF or MFF (or both) with her, had I played my cards better.
There was also a time I stayed with an old college friend and his male roommate while traveling. It didn't occur to me at the time, but In retrospect, I have come to believe they were probably lovers. We even watched some porn together the second night I stayed with them. It was straight porn and only went on for a few minutes, but had it gone on longer, I wonder if some bi or gay porn would have "accidentally" slipped in there to test my reaction. So there's a missed MMM threesome opportunity on top of the others above.
Although I've since broadened my scope, the reason I started writing erotica was to document my missed opportunities.