It sounds difficult at her current school with only 3 girls. Three is never a great number as at some point two of them will invariably side with each other and the third one will feel left out.
I think that is the sticking point - one of the girls is very dominant and the others tend to follow along. On a one on one basis the girls get on well together, but you introduce the third child and it all turns upside down!
She is going in for a taster afternoon at the potential new school shortly. We do know some of the girls (but not well) through after school activities. Most live within the village and it certainly would be easier for playing with friends after school.
Re the pros and cons: well, on paper it all makes sense to move her but... I just don't know! DD is ambivalent at the moment as she is currently happy but it will all change from September with the new mix of classes.
Ultimately it cimes down to how you would feel if they said she couldn't go to the new school you like?
TBH it probably wouldn't bother me that much and it would stop me having to make a decision!My
comfort zone is at the current school DD attends. I'm not sure my face 'fits' so well at our local school so I would always feel a little awkward and on the outside as far as interacting with other parents go. But then, it's not about me, it's about DD and her wellbeing and if this is better served by a larger school (even if I am then a 'social pariah') then so be it.