My boyfriend said he would do a threesome if i was open to it ???

He is playing games with you, he watch's to much porn and wants to play out a dream tell him to f--k off and find someone who wants you only

Tell him no...end of subject period, have a heart to heart and plan for a mutual breakup

that sounds like a red flag. But open up to him about it and see how he really feels :)

He wants to have sex with other women and is trying to manipulate you. You can try to salvage the relationship by seeing a therapist, but you'll probably be better off just ending it before he fulfills his fantasies and cheats on you.