Unstructured play dates of 3 are always a bit concerning to me, especially with girls.
They can go well depending on the maturity of the kids.
But girls have a hierarchy of friendships, and if one of the girls is not in a power position and doesn't know how to navigate the situation, it can get tricky.
My daughter is 7 so I understand where you are coming from.
She has a neighborhood friend "G". I agree as other mom's have suggested that neighborhood fiends are usually friends of proximity.
90% of play dates go great, and a lot of the time more kids merge in so that there is then a group of 5+.
But we had two tricky situations: the girls were going through a secrets phase and using secrets to be in the power position, and the other time "G" had a close friend over who was much higher in the hierarchy then my daughter and it became a 2:1 situation with my daughter being the 1.
I think these are pretty normal stages of relationship development. The only control I have in these situations is how my daughter handles herself. So we talk about how to listen to our gut/feelings to know when a play date is over, that it is ok to say that she is ready to end the play date, to speak up to the other kids, what behavior to tolerate, how to be a good host/guest......you get the gist.
These situations need to happen so that my daughter can experience when and how to use these social tools to better recognize good relationships and her sense of boundaries.
I think rather then always 'screening' these play dates, teach your daughter how to navigate them, and also 'host' more at your house.
If you haven't all ready, you might want to read "Reviving Ophelia" and "Queen bees and the wanna bees".