Jamie,
I went from 1 to 3 (twin boys) but we didn't know that we were going to have twins until after the first one was born. I know that I would have freaking out through the pregnancy if I did.
Ask for as much help as you can from family, friends, church, exercise group, moms group. I think that it is important to start off the little ones time with their new baby sibling with as little stress as possible. If you aren't able to give them as much attention as they would like then there will be someone else there to do so.
Once you could get around you can join moms groups or mommy and me class. Hook up with friends that have 2 and 3 yr olds and you could trade off childcare.(having playmates over would mean more children but believe me that as long as you have snacks and a place to play, they will be no trouble)
Everything takes a little more time. Once I accepted this I was able to deal with it better. One of my aerobics(helped the stress level alot)instructors gave each mom 5 min. per child to be late to her class. I was the only one that could be 15 min. late to her class and still allowed to stay. Most places won't do that so you will need to recognize how much time you will need with 3 and give yourself enough time to do things.
My three would play together and I didn't know how easy that made it for me until I had a baby boy when they were 12 and 14. Raising a little one so far apart is like have a single one all over again and it is way harder than having 3.
Anyway, just enjoy it if you are pregnant and communicate your fears with others around you and ask for help. Don't worry about your career at this time. There will always be time for that but the window of time is small that you can raise you children with love, caring and joy. Keep the added stress out of the picture if possible.
Crossing my fingers for you,
Evelyn