This being said, my child in Yr 3 has friends, but the boys that they want to run with, all seem to be unkind, and try to exclude them to a varying degree.
What exactly is happening here? Is it that your child has friends but would prefer to have different friends? So the children he wants to be friends with don't want to be friends with him? Or is your child being targeted above and beyond the usual spats between the boys?
This is a difficult one because, as posters have pointed out, children that age are not always polite, considerate and inclusive. They are often rude and thoughtless. It seems excessive to be 'raging' over this behaviour, even when it affects your own DC, because this is just what a lot of kids are like at this age. Many will hopefully grow out of it.
The inclusion thing is tricky. When commenting on previous threads re bullying/exclusion, I've always come down firmly on the side that, since we force children to go to school, schools must be inclusive and supportive places for kids. It is not acceptable for a child to have no one to play with because no one wants to play with them. It's up to the school to implement strategies to address children being excluded by their peers.
Having said that, some kids just don't gel and ultimately you can't force children to be friends with each other if they don't want to. And you say your child has friends, but just not the kids they want to 'run with'. So they're not been excluded as such if so, it's just the kids they'd like as friends don't reciprocate. Unfortunately, you can't force friendships between children.
If you told me that my child was being unkind to your child (anything physical, name-calling, pointedly excluding them in activities), I would talk to them and come down hard on the behaviour. But if it was simply that my child preferred to play with other children apart from your child (and this could be for many reasons), I'd be hard-pressed to know what to do about it and whether I should do anything at all.
One thing I would say is that I think it is harder for more emotionally mature and emotionally sensitive children to 'fit in' with their peers at this age. I mention this in case you come across the same issues at the new school. If your DC is mature beyond their age, your DC's peers may have a certain amount of 'catching up' to do before all the children are on the same wavelength. Ime even in Y3, the children who have the easiest time of it are the middle-of-the-road, easygoing, slightly boisterous ones who are happy to run around in circles in the playground shouting and chasing each other.