Verified answer

Tell him that the threesomes are over, sex is something that TWO people share, not three

Why don't you ask him to have a threesome with a "HOT blonde".

It sounds like you would like that or maybe he may rethink that sex should be between you two. (either way you get what you want) ;).

Just tell him you are simply not comfortable doing it and if he cannot accept this, then you guys have a problem.

Tell him exactly like that, you don't want to have threesomes and thats its non-negotiable. If he can't accept that, then leave.

just say no?

you're done! you are outa here!