Honestly, I've read a lot of threesome setups that rely on the surprise being purely who's at the door, so the ones that really stick with me hinge on an emotional or power reversal instead. There's this one fantasy romance where the established couple brings in a third to spice things up, and the twist wasn't that the stranger was an old flame or something—it was that the 'shy, submissive' partner of the original pair had orchestrated the entire encounter as a long-term test of loyalty, and was actually the dominant one controlling both of the other characters' actions through subtle cues. The physical dynamic flipped completely halfway through because the psychological groundwork was laid so carefully.
That kind of twist works because it recontextualizes every interaction that came before it. You're rereading earlier scenes looking for the hints. A purely logistical surprise, like a hidden identity, often feels cheap if it doesn't alter the relational chemistry. The best twists make the threesome about something much bigger than the act itself—reclaiming agency, executing a revenge plot, or fulfilling a supernatural bond's conditions. The erotic charge comes from the sudden understanding, not just the new body in the room. The book I'm thinking of sold me because the climax was emotional revelation, not just a physical one.