dont bring them to school at all.
at our preschool, one of the papers we filled out specifically asked if we would permit our information to be on the school list of all the parents addresses and phones? just ask in the morning if the school has one and i would call instead of sending the invitation via mail. just tell the parent specifically your son wanted to invite his 3 closest friends from school. if they dont mind, you will send them the info via email. it will make it known that others werent invited which will avoid hurting feelings, but also have a more likely chance the invited children will come. if you send it in the mail, they might assume you invited everyone and feel less obligated to come, not knowing your child really feels close to theirs.
if there is no list, just get to school early and wait outside for the parent. this will more polite than trying to do this while waiting or leaving. just ask them for their email so your son can send their child something. then just explain in the email that he wanted his 3 closest school friends there to let it be known everyone wasnt invited.
good luck to you