S.K.
I never had too much trouble transitioning when we had more kids. I just had my 4th a few months ago. After the first is born you learn to multitask pretty quickly, and for me the adjustment was quicker each time. I probably had the hardest time adjusting to my first because I was simply not aware how much my time would no longer be my own. Just try to anticipate needs as much as possible before feedings and include the older ones as much as possible. When the baby naps spend a little alone time with the older ones. Even "ask" the baby to wait while you help or play with either of your older ones, so they understand that everyone has to wait sometimes, not just them, but the baby too.
I think my biggest stress with 3 was getting in and out of the car in parking lots and such. You can't hold their hands and carry the baby too. Parking next to a cart corral became way more important to me than being close to the store itself. That way I could grab a cart and put the car seat in it and have a better handle on the older ones. Also when leaving the store it is easier to load up and get rid of the cart if you are right next to the corral.
Good luck and congrats on the new baby.