Three’s a Crowd

I’m a female and just started being in a relationship with a girl for the first time and I’m trying to figure out what is appropriate and what is not. The problem is my girlfriend is hanging out with a girl she briefly saw while we were in our “open” relationship phase. They go to the gym together, go to dinner, run all the time, etc. And this girl was heartbroken after my girlfriend told her she had to break it off with her because of her commitment to me. Now my girlfriend insists we all 3 go on runs together and hang out. I’m not comfortable with this because I get the vibe from the girl that she is still interested in my girlfriend. I don’t know what to do because my girlfriend claims she doesn’t like her in that way and that it’s strictly a platonic relationship. What should I so?