Help- sexual fantasy

Hi, so my boyfriend and I have been discussing our sexual fantasies and he mentioned that he would like a threesome with another girl. I am not into that but he is and he is enthusiastic about it. I have a few issues with communication that I’m working on but I am worried I will be disappointing if I shut the idea of one out even tho I never want one. He has never been abusive or anything but I’m a people pleasure. The thought of it is making me feel like I’m not enough, and he would prefer to be with someone else. So I just need a little help evaluating the situation. Thank you in advance 😊