Threesome?

I would just like to put out there that I do have trouble getting turned on for sex. I'm almost never wet and it always upsets me. The only thing that really gets me so turned on and in the mood is the idea of my boyfriend and I in a threesome. I don't know why and I try to push back the thoughts because I'm ashamed that my biggest fantasy is to share him. Well him being a male is obviously thrilled by this thought and we have the best sex when we talk about it. We have been together for 3 years and I was the one to initially bring it up and it's still always me. I do get jealous at times so he tries to avoid the subject until I bring it up. But last night I was in tears because I do want to marry him one day and I just feel like it's not natural to want to share him and I'm really confused. 😕