I wasn't one of the people involved and the pregnancy wasn't directly the result of a threesome, but a threesome instigated a saga that involved a pregnancy.

So a, b, and c were friends in one of my friendship groups. A and b were married, and c was a woman who worked with b, the husband. They worked in a field that required a lot of skill and knowledge. A, b, and c had a threesome that was meant to be a one off, but b and c carried it on behind a's back. They weren't very discrete and the whole friendship group knew so obviously a found out. B told a it was over but we later found out that it wasn't. While everyone thought b and c were over a decided to have revenge sex with someone none of us knew. She got pregnant but didn't know who the father was. A tells b she's pregnant and b tells c at work that it is actually over this time. C goes to the bathroom and trashes the place. Sinks off the wall, mirrors smashed, the works. When it's over, she doesn't remember a thing. Not just the tantrum, but the entire thing. She doesn't remember any part of her life, in fact. You see the entire time anyone in her life had known her she'd actually been possessed and she suddenly became free in that bathroom. Unfortunately she didn't remember any of the knowledge and skill required to do her job so she now works in a shop and doesn't recognise any of us.

Both of a's kids look like one another so they're likely both b's kids.