A threeway will only work if all three parties consent and are agreeable to it and there is the mutual attraction for all three. From your comment above about your roommate being jealous easily, that is not going to end well. They may have suggested it on a whim but when it turns serious, I doubt she's that willing to share and as I said, it could end badly, not for them but for you.

Tell them thanks but no thanks or just say you'll think about it.