For fantasies including a third person, I sometimes believe that a what is good for the goose is good for the gander view is required.
If it is another guy a promise should be made between the women and man (presuming here) that whatever the woman says the man has to do. As for the lover, the same rule applies.
Then to make sure before the “bedding” begins, start with the guys on the couch (like stated at the end) where maybe she is between the two of them, and then she gets up and says … before either of you “touch” me … I need each of you to touch and keep and get each other hard while I play with myself in front of you.
After that is done, and presumptively the woman takes the lover in …. state … you are not allowed to orgasm or touch yourself while we make love, have intimacy or get each other off …. your fulfillment comes after mine ….