Threesum/advice

My boyfriend and i have been together almost 3 years. He wants to have a threesum, i dont want to. He told me hes going to have one and i keep telling him no hes not, because in my eyes its cheating. He says it isnt cheating, and that he wants to fuck them. Im not okay with it, all it has been between us is a fight. What the hell do i do. Im so heartbroken.

Or he wants to have sex in the same room, same bed, with me then swap partners.