Welcome to the forum!
My partner and I started out our poly life with a triad. It was absolutely delightful. I wish you the best of luck!
Rule number 1: give it time. If it's meant to be, then you have many decades to explore, change, and be comfortable with each other. If it's not meant to be, rushing things doesn't help! If S is comfortable being sexual with you while in a threesome now, it's possible that in a year or five she'll be comfortable being sexual with you elsewhere.
Sexuality is fluid. Over the years we change as individuals, in every way.