Relax......I am working on adding a 4th to my family. I really felt the hardest transition was from 1 to 2 kids. Going from 2 to 3 kids was a lot easier, because you already have learned how to divide your time/attention. Be sure to not always ask you oldest to "help". The middle child will feel more important and "needed" if you have her help, too. My boy and girl were 20 months apart and he loved getting me a glass of water when I sat down to feed my baby girl, picking out her clothes for the day (with some help of course) or getting the diaper out of the holder while I changed her. The other life saver that I had was a sling. That way, when baby was fussy, I could still have two hands to help my other kids. Also, it helps for outings when I need to hold both of my older kid's hands while crossing the street, etc.
Congrats and good luck with the transition.