Threesome

My boyfriend wants a threesome with me and a girl we know, I have a bit of history with this girl (just touching/kissing) but I suggested after bumping into her one day we should have a threesome.... he immediately took to the idea and will not leave it alone. He says he could see the chemistry between her and I and despite me saying no he still wants to do it. I don’t think I can see another girl on him and pleasuring him along with the reason I will probably get jealous and not see the relationship as the same afterwards and we are in a serious relationship together (child, living together, etc) he won’t understand I’ve said no and it’s getting to the point where I might as well say yes because I know there’s no emotions attached and I do really like this girl. It’s different for him he doesn’t see it like I do. Should I just agree and see how it goes? I’m shocked he wants to be with someone else to be honest since he “adores me” and im “the love of his life” blah blah blah. Anyway I want to please him sexually and let him get to know what he likes and to explore things but at the same time I think i already see him differently. Please help... no hate just advice!!!