There's fun stuff for three year olds in all 4 parks.
At MGM, there's Playhouse Disney, Little Mermaid, Honey I Shrunk the Kids Playground, Muppets, Beauty and the Beast.
At EPCOT, there's Figment, Innoventions where you can play Toontown and other computer games, Kidcot stations in every WS pavillion, Kidcots in some of the FW locations, the maze in England, Rio Del Tiempo in Mexico (think Small World Mexican style), and if tall enough, Soarin (lowest row works great...not scary. My youngest hates the Peter Pan ride but loves Saorin', etc. Plus there are character met and greets all over the WS...many with very little lines or no lines.
MK, you prob know all these.
At AK, there's the Boneyard, take the train to conservation station and the petting zoo, triceratop spin, Kilomanjaro safari, Festival of the Lion King, Camp Minnie Mickey, Pangani trails, etc.
We started going to WDW when my youngest was 2. She loved all these (except Soarin...she just tried that of course when she was 4).