I don't think many people have to worry about keeping track of body parts except for serial killers and writers. Makes me think of the opening to the TV show Castle. "There are two kinds of folks who sit around thinking how to kill people: psychopaths and mystery writers."
I've only written one menage so far. Private Pleasure is a F/F/M menage. It was a bit easier considering one guy was involved. The menage short story I'm working on this month has three men! There are way too many hims and his and he's. It is rather confusing.
I've asked other menage writers how they manage to keep body parts straight and it seems to the key is a lot of editing. So for the time being I'll just keep writing and then we'll see if I can get all the body parts to go where I want them, lol.
Have you written a menage? If so, how did you deal with keeping track of body parts?