I've seen this so many times as a teacher and as a mum and even as a child myself! Two will always gang up on one to make her feel left out. The harder piggy-in-the-middle tries, the more fun the other two have moving the goalposts and playing their power game. It always ends in tears.
Your DD needs to make new, better friends - not all children behave like this - ideally one bosom friend she can just enjoy being with and not be anticipating the next hurtful remark or snigger at her expense.
Don't talk to the other mums; it will come back at her in nasty ways. Instead, why not empower her by rehearsing some strategies? She needs some phrases to deploy when the get nasty with her, like "I'll just let you two get on with it then - there are other people to play with!" and teach her to reach out with a phrases like, "Can anyone play? Can I join in, please? This looks like fun."
When she's hooting and hollering round the playground having fun with a different bunch, the mean girls will lose their power.
I'd be having a word with the teacher to identify a more welcoming group who could be primed to invite DD into their games, ideally a mixed sex group. Boys' play is generally less talk-based and more active, which would give her a nice break.
I never allowed DD2 to have two friends over at once at that age because one of any three always ended up crying on the stairs.
The rehearsals we had trying out what different people might say and how my child could respond stood all mine in good stead later in more complex relationships. I still occasionally get asked for advice in handling sticky situations and the youngest is not far off 30 now!
Best thing I told them was, "If you want to have a friend; be a friend."