Threesomes HELP

My freaking bf of 5 years and father of my 2 children has apparently "always wanted" a threesome and i know itd BREAK ME to see him have sex with another woman. (We have only ever had sex with eachother) he said he would let it go bc hed rather have me and no threesome then lose me but now he wamts me to talk to my bi friend about him and make them jealous they cant have him and brag about how great our sex life is. Me and this girl ALWAYS talk about our sex lofe and fantasies and i DO brag about him but its like...no matter what he needs some type of "third wheel" wether its a threesome or this texting to make her "jealous" and i am just so emotional about it and it makes me feel like im not enough for him...am i wrong to feel this way? Has anyone experienced a threesome or what im going through& did it end badly?