My 3rd was also my easiest baby, he turned into a regular toddler though! My other kids were 6 and 2 when he was born, my oldest was a HUGE help and honestly how we made it through every day. I could put on a movie, hand her the baby and go take a shower, eat, or put DD2 down for a nap, she'd sit next to him in the car and keep him happy.
It is chaos, they are 8, 4, and 2 now, I've got one more coming this summer. Someone always needs something, the fighting as they get older. Something always seems to be happening with at least one, illness, phase, etc... Balancing needs is hard, and frankly sometimes impossible, one might feel left out, but you just can't make everything right/fair for everyone all the time. DD1 started school a couple months after DS was born, I missed out on a lot in her classroom. Volunteering, class trips, I still rarely get do anything at her school. The older she gets the harder it has been to juggle the siblings, a 8y and a 2y can play together for a bit but she also deserves the chance to do older kid activities without always having a toddler that has to join in.
My 3rd never seemed optional, I needed that child, my 4th was the optional one in my mind. In the end, baby decided to come before we had made a decision, one of those meant to be things I guess. I've enjoyed the dynamic of 3 children. It is different, we spilt up the family often now, and I do things like use sitters far more just to accomplish basic tasks or to help with that balancing act. I was watching them today at the beach, in a line, biggest to smallest on a treasure hunt. They looked like a flock of ducklings out there. Those are the moments that make all the craziness worth it.