My Partner's Ex Girlfriend

since two years now I have a boyfriend that I really like and communicate really well with. The only difficulty that I had with him since the beginning was that his best friend is actually his ex-girlfriend. After a year though, I started getting to know her better. Then I started having sex dreams about her. I never had an experience with a girl before and that made everything even more new, complicated and exciting for me. My fantasy goes always around a trio between all of us, but that's something I would never dare to share with my partner or her!