Warning: The following post is long, definitely TMI, and though I’ve omitted the most explicit details, reads something like a bad letter to Penthouse. I don’t care. The OP has asked for threesome stories, and I have one. I’ve very much enjoyed typing it and I wish to share. I swear every word is true. Feel free not to read it. 
I was 19 and pretty inexperienced sexually (a couple fumbling encounters and no real relationships to that point).
My randomly-assigned dorm roommate (“C”) was a few years older, having done a stint in the Army before college, and was comparatively much more sexually accomplished and confident.
We got along very well in a raunchy 80’s buddy-comedy movie sort of way. Even though he turned out to have all sorts of problems, I looked up to the guy. I was a geeky, gawky kid and he took me under his wing – think Pinto and Otter in Animal House.
C began dating K, a girl from the next building. She had apparently been fairly inexperienced as well, and he delighted in telling me about all the various things he’d gotten her to do with him as their relationship progressed. I didn’t mind; I kind of enjoyed the vicarious thrill.
One day, C told me he’d been working on selling K on the idea of a threesome, and she’d decided she wanted to do it – this weekend! So, did I wonder about the psychological implications on the dynamics of our respective relationships? Um… no. I was 19 and suddenly handed a golden opportunity to get laid. That alone trumped any hesitation I might have had about another guy being there. In fact, in a weird way, I think C made it less awkward than it probably would have been if I were alone with a girl at that time of my life. (More on that later.)
On Friday night the 3 of us went out for pizza and not a little beer. As we got drunker, she got flirtier, and we were all getting horny, so it was time to head back to the dorm. We sat on the bed with K in the middle, and she alternated making out with C and me, with the touching and groping getting heavier as we went on.
At that point, C became the “director” of the proceedings. He sat in a chair and essentially narrated K and me through the act without yet joining in. I admit I enjoyed being told what to do, and it also took a lot of the pressure off a kid who didn’t really know much about what he was doing.
After I finished (which – are you surprised? – was quite soon after I’d started), C and K did the deed as I watched. Then, after we all regained our energy, we started experimenting in various activities involving all 3 of us at once. The mood in general was really playful and loose; at the time I don’t recall there being any weirdness at all. C and I didn’t do anything with each other, though there was some incidental touching of parts from time to time. Again, surprisingly, that wasn’t weird.
The sleeping arrangements? Well, it was a dorm room. I climbed up to my top bunk while C and K slept together below.
In the harsh light of the next morning, there were no regrets. Quite the contrary, in fact. We arranged our mattresses on the floor to allow for better freedom of movement, and kept at it all day and into the night. That remains one of the wildest weekends of my life, and I truly believe it helped me on the road to sexual maturity and gave me some confidence – even a bit of swagger – that has served me well over the years.
Afterward, I was looking forward to our little arrangement becoming a regular thing. That didn’t happen. A few days later, I walked into the room to discover C and K having sex on his bed. I played it cool and was about to leave them to their privacy, when C chimed in with “You can go ahead and jump in anytime here, Wheelz!” K did not appreciate that, to say the least. She flipped out a bit, yelled something at him about respecting her feelings, hurriedly got dressed and stormed out. Yes, it was kind of a dickish thing for him to say, but not entirely out of left field given the things we’d already done together. Looking back, my guess is that K had some second thoughts about what she’d done, but didn’t want to come out and say so. So she blew C’s comment out of proportion to avoid a repeat.
But she didn’t feel bad enough about it to leave our lives entirely. C and K had an on-again-off-again relationship after that, and she and I briefly dated (with C’s blessing) during one of their off-again periods. She definitely continued to enjoy sex – just with one guy at a time. We talked about our weekend occasionally, but there was an unspoken understanding among all 3 of us that it would never happen again.
I never saw K after college. C and I stayed buddies for a few years but eventually drifted apart. If I had to guess, I’d say we all enjoy remembering what happened that weekend.