Our boys are in Kindergarten and in three different classes. Luckily in Kindergarten the school rule is to please invite the whole class. Our boys do get invited seperately to parties and the ones who do not get invited do not go. We try to focus on it positively and treat them as individuals letting them know that over time they will each get seperate invitations. One of our boys was so excited when he was invited alone he slept with the invitation for two weeks.

I would send the two to the party, don't punish them and do something special with the other triplet giving him some one on one time. Everybody wins. Good luck.