Your request is very sensitive to me because it "hits" home. I actually felt your motherly pain. I went through the same thing you are going through now. My son is 7yrs old now and he was in that situation at preschool and even at 7 yrs old he still goes through this. At preschool, I also did like you and talked with the teachers so that they were aware of the situation and they were very attentive. Some children are product of their environment and I believe they are simply not being taught at home good social skills. I encouraged him everyday, I told him he was loved and that he was special and I let the teacher and I still let people know that he is loved and he is special. Eventhough, your son is 3 yrs old start teaching him to control his situation, you Mom and Dad are going to have to do this so that he can stand up for himself. It's not too soon believe me. Tell him that if someone is mean to him that it's okay to tell them that they are "mean" and he doesn't like people being "mean" to him.
I believe you did the right thing to take him out of the daycare because sometimes children know things aren't right but cannot express what's really happening to them but they show it and just because the preschool was highly recommended doesn't mean it is really up to "Par".
We are a military family and we live here on base with a lot of children my son age and we definitely again encountered this situation where my son was the minority and the little boys had their little groups of who they played with. My son would simply come home and tell me that he has no one to play with and A. B it would break my heart to see him cry. I would just go into the bathroom and just shed tears but I wouldn't let him see me but I would encourage him, I and my husband would make it a family day and again remind him that he is loved and that he is special. Find out what he likes to do because even at 3yrs old he is beginning to have a talent or an interest in something maybe he likes to draw, sign him for soccer or baseball now that the weather is warm. I'm sure he would like that. A. B this is not the first time you and your husband are going to encounter this problem with him, there will be many more but keep encouraging him and tell him you "love" him everyday and that he can do anything he wants to do!
I know that my son is a little different than most kids because he was born singing in the womb and he sings everywhere he goes. But, I love the little "singing bandit" God gave me..:))