I would caution against getting involved too soon. Rebound relationships rarely work out long-term, and if she is just getting out of a relationship AND the two of you have recently gotten out of one - that's a whole lot of rebounding going on!
Not saying that talking to her and letting her know that you are interested in seeing where things could lead would be bad, but jumping right into a relationship could be.
Why not just try to casually hang out for a while, make sure everyone is in a healthy space, and then start dating?
I would just say something like, "Hey, I know we really enjoy spending time with you and we think that we could really fall for you. I know it's kind of a bad time with you and boyfriend just splitting up, but if you feel the same we would love to see if there is the potential for something between the three of us when you feel ready. If you don't feel that way for us, we understand, and we definitely don't want to lose your friendship"