There is a lot on unicorns here that might be helpful. It sounds like you might have some questions as to what might play out if you were to pursue this. If you do a tag search for "unicorns" you will find many threads.
As to approaching her on this, it sounds like she is sending a clear message, but maybe its just fun and playful and sexual... are you interested in a committed poly fi closed triad, a closed vee, an open poly relationship with her... what are your thoughts? It would be helpful to know. Then you can start from there and ask her what her thoughts are.
It seems important in any poly relationship that there is some openness to fluidity, to things going where they may and not having expectations and assumptions from the get go... at least until such time as there is movement towards that... it seems many people get hurt when it becomes evident that the bonds that once were for all change to other combinations. Being aware and taking the time necessary to evolve is important I think.
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