You are all far too involved.
Your daughter needs to learn and deal with rejection, these things happen.
We have all been children, and I don't think any adult can honestly say they were not victims of some kind of "we don't want to be your friend" situation.
You cannot force these children to be friends. As PP said, all they'll do is put on a show for you, and then reject your daughter anyway. Prehaps even more so as the resentment of being forced builds.
Of course it is unacceptable if your daughter is being bullied by these children, but it sounds more like they don't like your daughter, and don't want to play with her, and as horrible as that feels to you and your daughter, that cannot be changed or forced.
I would strongly recommend that your find your daughter a 4th friend, or encourage other friendship groups/children - teach her the skills to cope with rejection, appropriate behaviour and to create and form new friendships.
Children's circles change and form all the time, these two girls leaving your daughter out may not last, but forcing an issue will only drive them together and make life worse for your daughter.
I hope this isn't coming across harsh, I understand how awful it is to see your child hurting and not feeling welcome, but please take this as an opportunity for growth for her, rather than a negative.