This question made me giggle. Not in a mean way, but just remembering how we worried about how the kids would respond to a new baby. Honestly, the kids did fantastic! We always told them that we would be bringing "their" baby home and that the baby couldn't wait to meet them. Then we would, and actually still do, tell them how proud we are of them for being such a good brother or a good sister.
As far as being a parent and transition... for us it was 0-1. We thought we needed everything that people told us, and what the books said we beeded, and we'd try to give her everything she wanted. Plus it was, of course, a life-changing seriously LIFE-CHANGING experience...beyond anything imaginable! All of a sudden here we were responsible for this new little being. Having 2 was okay, we each could watch 1 of the kids when we went out. I would say 3 brought back a little bit more challenge and humor, because now we were outnumbered and we were thinking we knew pretty much everything as far as having children went! HA! LOL.. but, this also made for for some fun memories. Like all of a sudden I would forget that I needed to grab the diaper bag or bottle, because I was busy getting everyone else loaded up in the VAN! Then there was accepting the fact, that we needed a VAN... LOL which I guess I never thought I would enjoy driving on a regular basis!! Or forgetting that I couldn't take the other kids to the doctor at 1 in the afternoon, because that was naptime for the baby! (Our rule in our house is NEVER EVER wake a sleeping baby...even if they fall asleep in the middle of the living room! )Not to mention that we were the family that now with 3 kids, were perpetually 15 minutes late for everything. LOL...now with 5 kids, we try to keep the lateness under 1/2 hour and call it "mission accomplished" if we succeed. :)
I'm not sure if this exactly answered your question. But I guess the short answer from me would be...0-1, then 2-3, but the good news is after 3, the rest just float in so easily...because by then, you've become a good juggler, figured out how to deal with different personalities (which, they are always different), and are more confident in being a mom( or dad) and the word "no" just flows so much easier from your lips ;) The best advice that I could give to you, is just to remember to have a sense of humor...and don't stress about all the little things. Things and the little people in your house are going to get dirty, make messes, and make you crazy, but at the end of the day, there is NOTHING better then having a child tell you that they love you, your the best mom (or dad), or just come and snuggle with you. With 3, you just get that much more and are truly, truly blessed :)
Congratulations to you and your family and I wish you many moments of happiness and blessings!