Honestly things like this happen all the time. Try not to spend too much time thinking about it. With my own kids, I try to watch social interactions, but I also try to see how they will navigate on their own. Sometimes if my kids are the ones being standoffish I let them know that the other child wants to play. Sometimes pointing this out leads them to include the 3rd child. I also talk to my kids often about how it feels to be the new person or person left out, and talk about trying not to hurt other feelings. I don't typically make them play with kids when they don't want to, but I hope that they will learn on their own to notice when other kids want to play with them.
In this particular situation it sounds like the other 2 girls might already have known each other. Sometimes if kids come together or meet at a play area, they want to play with each other. 3 can sometimes be a crowd.
Since you all (the other 2 girls and your daughter) had the right to be there, I don't think you have to keep her away from the other kids all the time. But I would probably try to distract her and help her learn to play on her own too.
I think you handled the situation fine. :)