I've been in two actual threesomes and two fizzled attempts at them.
My first-ever threesome was when I was sixteen, and it took place in a hot tub at a friend's New Year's Eve party. That sentence alone should be enough to tell you that it didn't go well. For as arousing or sensual as hanging out in a hot tub might be, it's not really the best place to do your naughty dance. Still, hormones were bubbling, and the two girls I was with - one of whom was my girlfriend at the time - were both eager, so we gave it a shot.
Unfortunately, the entire thing only lasted about five minutes before a combination of awkwardness and physical discomfort brought it to a premature end. We'd tried "group kissing" - which my girlfriend had insisted was a thoroughly pleasurable endeavor - and a lot of mutual touching, but nothing was especially stimulating for anyone. My girlfriend got frustrated, our mutual friend got bored, and we all decided that maybe playing Ping Pong in the garage was more fun after all.
The next time that I had a threesome - if you could even call it having one, which I wouldn't - went even less well. I'd made plans with an acquaintance to visit her apartment, get drunk with her and a random girl that she'd allegedly found on Craigslist, and then see where the evening took us. There was only one catch: Since the young woman hosting the event had already started drinking, I'd have to be the one to pick up our third partner. To make a long story short, it turned out that the two of them had already known each other, and that I'd been used as a free taxi service. I wound up leaving (at their request) within ten minutes of arrival.
Now, with both of those failed attempts under my belt, you'd think that I'd have an aversion to threesomes... or maybe you wouldn't, I don't know. The point is that I tried again (with different women, though one was, again, my girlfriend) several years later, and it actually turned out to be a really good experience for all of us.
We started off by playing "Strip Scrabble," the rules for which we kind of invented on the fly. Basically, any time someone scored fifty points or higher, the other two people would have to remove a piece of clothing. That requisite point value declined as the game went on, and by the time we'd used all of the tiles, we'd resorted to offering silly dares (since we'd run out of clothes to remove). At one point, I was tasked with imitating a Star Trek character while doing the "helicopter dick" across my bedroom. Not long after that, we all adjourned to my bed and jumped into more direct sexuality. We had some laughs in trying to figure out the mechanics (and at one point - completely by accident, I swear - I went in through the wrong entrance), but on the whole, it was a lighthearted and enjoyable evening.
I had a second threesome with the same two girls a few months after that, and it was even better.
I know you were probably expecting a story of things going catastrophically wrong, but honestly, it was as simple and casual as having a group conversation. My opinion on the matter is that everyone needs to be sane, honest, and uncomplicated. As long as you're all just there to have fun, even the most awkward moment can be a building block for an entertaining memory.
Just be sure you get your angle of entry right.
TL;DR: Fizzle, fizzle, fuck, fuck, twice misfortune, twice in luck.