Hi Melissa! For me, a specific answer depends upon when you are planning to visit. Certain times of year, such as summer and major holidays, you would need at least two days at Magic Kingdom just to see most of those attractions. During light attendance times, however, you can cover everything at MK in one long day.
I asked my 7-year-old DD (a WDW verteran) how she would spend your passes. She votes for one day each at Magic Kingdom, Epcot and Animal Kingdom.
Don't try to see everything--three days simply isn't enough. My advice - Do some reading before your trip and make a list of must sees, but save time for a few extras. You're sure to whet your appetite for future trips.
Have a wonderful trip!