Should I have a threesome?

My boyfriend and I have lately been thinking about having a threesome (ffm) he has been really supportive and said won’t do anything unless I’m 100% okay with it and I really appreciate it, but here’s the thing: sex with another girl has been my fantasy for a long time and I can see myself doing it however when it comes to a threesome I’m only really into it when I’m horny and turned on, but when the feeling wears off I just don’t see myself doing it with my bf and another girl idk guess I’m the jealous type?? I’m really curious of what it’d be like and I’ve read a lot of stories online on how girls thought they’d mind but at the end of the day they really enjoyed it too so maybe that’s what’ll happen? Maybe I’m just overreacting??