Communication - Not Combat
I certainly would recommend looking at this as a growing experience - not as a call to arms. I don't know you or your partners so I am going to presume that all three of you mean no harm to one another. Any harm she may have caused against you (real or perceived) is almost certainly unintentional.
I find that when I take an approach of humility, making it known that I am having some emotions flare up, that I ruffle fewer feathers and real communication can begin. When I get those jealous/envious type feelings it is certainly my responsibility. In my relationship(s), ownership of any kind is not the goal, so why would I be having territorial type emotions if I am not (on some level) trying to control/own my partner? In that, I express that "when you said this, I had a jealousy type emotion come up. I'm not sure exactly why or if it is reasonable. Most likely it is something about my worldview I need to look at"
Just talk to her, tell her how you are feeling. If you are honest and open yourself up I would wager you will be surprised by how helpful her response will be.