Congratulations on your little bundle and color me jealous! This trip will be our first without an infant in a long time (youngest is now 2). These are my thoughts:
All of your children are very young and you should be planning a more leisurely pace. Perhaps your infant daughter will actually force you to pace yourselves.
Your daughter will be happiest in your arms - no matter where those arms are: standing in line, waiting at dinner, at a show, on "it's a small world".
There are many debates on this, but there is no better way IMHO to keep baby close, happy, safe, and your hands free than in an old-fashioned baby sling. My husband refused to use this with our oldest. But when number two arrived, he became a true believer. He's now been sling-wearing babies & toddlers for the last 5 years! (So has mama.)
Your 3 year old will not be able to ride the same attractions as your daughter. So baby should not impact overall touring plan.
The joy (and later photos) of your daughter at 1 character breakfast will make it all worthwhile!
If you leave her behind, what will she think about that in 10 years? Disney is for families!
I am sure I will think of more reasons why families should vacation together (or wait a little longer). But I'll end this long post now. Good luck with your decisions and planning.