Hi Elizabeth!
It all depends on what you want to do when you get to Disney. There are 4 major parks, and 2 Water Parks. Just factoring in the theme parks: Magic Kingdom is generally 2 days, while Epcot, Animal Kingdom, and Hollywood Studios can take 1 day if you start early.
My recommendation: decide on where most of your "must do's" are. If you find most of them are at Magic Kingdom, I'd set aside 1 day for that park (since you only have 3 days, don't worry about setting 2 days aside if you go here)...
In terms of fun for pre-schoolers, Magic Kingdom and Animal Kingdom are my top 2 destinations (I have a 6 and 2 year old).
Magic Kingdom has all the shows, parades, and a number of child-centric rides.
Animal Kingdom is a favorite, just because my children love animals, and there are some great attractions (Kilimanjaro Safari), a chance to see and pet animals, and good "hang out" spots, like the Boneyard.
Possible itinerary? 1 day for Magic Kingdom, 1 day for Animal Kingdom, and a "floater" for Epcot, Hollywood, or maybe even one of the Water Parks.
Have a blast when you go! It's just around the river bend!