Its a triplet parents worst nightmare, really.
I only have triplets, so most of what I;m about to say about singletons is conjecture and assumption, but here goes:
I imagine that when you have three kids of different ages its easy when only one of them is invited to a birthday party. Any younger child is probably interested in where an older sibling is going, but is easily refocused. Older children probably just don't care what a younger child is doing, but to the extent they are invested, I'd think its easy to explain to them. After all, they are probably in different schools, or at least different grades. They have different teachers, different classmates, and while they may share some friends, those are largely different as well.
Not so with triplets
When you have three kids all the same age they attend the same school; often in the same class (as ours do). So when only one of them receives an invite, as our daughter did, its hard not to feel slighted. After all, they know, play and enjoy the company of the birthday girl just as well, or better, than Sadie.
Ok, so its not exactly the stuff of monsters hiding under beds, abducted children held in terror domes and losing all your teeth (all nightmares I've personally experienced).
And as it was, this is a "girl's only" party. I won't get too far into whether "girls only" parties are a good idea at this age. Mostly because my thoughts on the topic are initial reactions and rudimentary at best. But in my experience girls and woman can like vastly different things and having a "girls only" party only serves to enforce stereotypes. Same goes for boys. My one boy loves Disney Magic Clip dolls, as you may have heard.
But at least the "girls only" status made it easy to explain to our boys. Thank goodness we didn't have two girls, one of whom was excluded, instead.
UPDATE: An impending winter storm promises to save us from this mess by introducing a whole different mess. So we get a week worth of salvation.
The boys actually handled this really well. They wanted to get mail more than invited to the party. Oh, to be a four-year old. It helps that we promised them we would do something fun.
I think one reason I can't get worked up about some of the things Alex mentioned in her comment stem from the fact that we do a bad job of fostering individual time for each of the kids. Its something we were told early on: you have to make time for each one to be an individual, because otherwise they get lumped as "triplets" and struggle with an identify. We have had some luck in that each one is so unique that that they have formed their own identify pretty easily. Still, we don't do enough to get them out alone with just Mommy and Daddy so that they can develop their own sense of self and have that unique time that singletons enjoy.
THAT is probably my biggest parenting regret four years in.