there are not actually canonical tags for specifying how many people are having sex, aside from “Group Sex”
“Solo Masturbation” is synned with “Masturbation” (despite the fact the latter includes “Mutual Masturbation”) and “Couple Sex” is synned with “Sex” (which includes “Group Sex”)
this ontology is a mess
🐊
apparently just shipping three people together makes them a “threesome” in AO3 terminology