This is super long, but I kind of glanced over it, and as a woman who's had 3 threesomes, you're a bit off.

I've only had them with FWBs, all MFF, so maybe that helps. But I've only ever felt left out when there was a lack of chemistry, which happened during my first one. My second one was with two partners I'd been with before (they'd also been with each other.l before), and it was awesome. I don't think anyone felt left out (perhaps the guy slightly, since the girl was a little more focused on me), we all cuddled just fine at the end, and nothing felt awkward. My second one was with the same, and a girl he knew (I only met her a few minutes before we hopped into bed together). I'll admit that it was a bit more awkward, but I didn't feel left out, and it was fairly similar to my previous one. Another thing that may impact my experiences is that I enjoy watching. I think it's incredibly hot to see someone I've been with fuck another person. I'm sure not all people have that kink.

Chemistry is key, IMO. And not having anyone be more involved with one participant than the other also helps. Again, with my first one, the guy and the girl were a lot more intimate with each other, and it did bother me. I felt like a guest star, even though I'd been sleeping with the guy on and off for several months. If you're giving everyone the same amount of attention, it makes it feel more threesomey (rather than a 2.5-some, which is what I consider my first one to be).

Your write up doesn't seem so bad, but I think it's more directed at romantic couples introducing a third partner.