My oldest was an only child for almost 6 years. We did have the step siblings every other weekend more or less, but they weren't my kids so they weren't a threat to Mommy time. December of 2005 there was an intruder to the monopolized Mommy's time. The first born was jealous, he was insecure and he was feeling very left out. He knew the baby was coming. He was old enough to understand, but despite being reassured many times a day it was a very rough transition. He wanted nothing to do with this new intruder. No pictures with the baby, no touching the baby, wouldn't help mommy at all with the baby. Nothing!
Eventually the second child got older and started interacting. The first born started to warm up to this intruder. They started playing together and really getting along for the most part. They are friends and brothers and get along better than any parent could ask for. Then came the next intruder.
The third one is different. First off this intruder is a girl. The oldest now used to being a big brother is interested in the new baby. He will come and cuddle mommy while she is holding her. He will stroke her head. He still doesn't really want a whole lot to do with the baby, he won't hold her or anything, but not feeling completely left out this time around.
The now middle brother on the other hand can't get enough of the new baby and at the same time wants Mommy all to himself. Just being three he wants to help, but really isn't big enough or gentle enough to really be a help. He wants to hold the baby and get things for the baby, but HE is still the baby and HE still wants Mommy's undivided attention. He is still too young and doesn't have the patience yet to understand that Mommy is feeding the baby he has to wait a minute. Mommy is changing the baby he has to wait a minute. He just doesn't quite get it. He will eventually.
Getting used to having three kids isn't all that much harder. It is just juggling time for each one. The baby of course monopolizing all the time. Only being 3 days old things are still very crazy, but the routine is being set. Thankfully the oldest child has 5 days off school. Plenty of time to get adjusted to a new schedule. We will figure it out together. The three kids, Mommy and Daddy will get the routine going and everyone will have plenty of time with the desired parent. The family is complete. We have a full house.
The cats on the other hand......