If you thought the girls were generally really nice and this was new behaviour for them or there was one bug specific incident where you had reasonable proof I'd speak to the mums.
As is stands, I wouldnt. You dont think the girls are actually nice, what do you thinks going to happen? They will say to their kids 'x's mum has contacted me, they say you're not being nice to x and leaving her out'. The kid will most likely say 'that's not true, I'm nice to her, she always tries to change our game / gets jealous and play with us separately etc' and the mum will of course have to believe her child. If you dont like these friends then they've probably always had these tendencies and realistically, do you think when they are together as a three, they are going to change their behaviour?
I would speak to the teacher as when it crosses into bullying behaviour then they can look out for it. But just wanting to play two of you isn't really bullying (as excluding one child from a larger group woukd be) although its still not very nice.
I'd explain to your daughter that you will speak to the teacher and see what she suggests (eg other groups for her to play with) and review it in a couple of weeks. But that this may stop them being nasty to her but it wont stop them not wanting to play with her, and forcing them to play with her (whether through a chat with the teacher or her mum) wont really work because they wont mean it and they will still be the same people underneath who treat their friends badly as soon as someone they want to play with more comes along.
And as other people suggest I'd try and concentrate on building friendships with other children.
The only way I'd mention it to the mum is if she contacts you to arrange a playdate, I'd reply something like 'oh I didnt think the girls were getting on that well at the moment, my daughter has been a bit upset over a few fall outs recently, has yours not said anything?' And see what she says.
I am actually in a similar situation at the moment, my daughter is only in reception and has got involved in a three and they seem to play nicely when its 1 on 1 but there is one girl who likes the power play of choosing which of the others is 'allowed' to play with her at lunchtime and encourages the other to be nasty to the other. At this age it's more overt things like pushing the 'outcast' over or not letting them sit next to each other, so the teachers have noticed and brought it up at parents evening though were careful not to blame anyone. I havent really done much else as my daughter isn't really upset by it as she has a couple of other friends in other classes and will just go and play with them. But it's so frustrating when she acknowledges that this other girls behaviours 'arent kind' and says she would never do that to a friend but on the other hand always rushes to play with her as soon as she is allowed to because 'she likes her'. I just hope in time she realises that continual bad behaviour means this girl isn't very nice and she can find better friends who make her feel happy