It is perfectly normal for a 3 year old to be hitting and pinching. At this age, kids still play side by side not "with" each other.
With most kids (including those with disabilities) the best way to approach this is to try to give the child a different way of expressing themselves. Even more so if the child has language or social issues.
It is also important to understand WHY the child is doing this. It could be that the other children are causing the behavior (taking his toys, name calling, ignoring him etc..)
When my youngest son was 4 years old we were having problems with him hitting a couple of girls in the daycare. NOTHING anyone tried stopped it. Then one week one of the girls was gone on vacation with her parents and we had NO ISSUES at all. The next week, I then discovered the cause of the problem. The girls in question from the minute my son stepped into the classroom were constantly berating him until he wore down and lashed out (we had kept asking the school what the girls were doing to cause this and kept getting told they weren't touching him). After I overheard the girls laying into him I got the director involved and they finally realized that the girls taunts were just as important to deal with as my child's reaction to the taunts.
We couldn't teach our son how to handle the situation any better until we understood what was causing the issue (in this case, we simply told him each time the girls started in on him to go get a teacher and told the school to keep the girls away from him) and your daughter can't teach your son different ways of handling things until she understands why he's doing it.
She's going to have to be diligent about watching her son, but also diligent at pointing out to the OTHER parents the inappropriate behavior of THEIR children. Any parent who tries to tell me their kid has never hit or pinched another child is either lying or never had their child around another child.
Yes, we have also been on the receiving end of those hits and pinches (my oldest child in his first day care had a girl that had a propensity towards biting him yet they adored each other!) and can understand the frustration, but the child is 3 years old, not 13.