It’s not your job to get these two women’s friendship back on track. In fact, you can’t. It’s really their choice to be friends or not. Clearly they both liked you and when you chose one over the other, the dynamic between the three of you changed.
Enjoy the romantic relationship you have with one of them, and be kind and empathetic to the other one — but be realistic. If they don’t want to be friends, that’s between them and is their business.