We took my DD for the 1st time when she just turned 3. She had a ball. I'll try to give a park by park list of what she liked
MK-Dumbo, Peter Pan, It's a Small World (she LOVED it), Mad Tea Party, Carousel, Mickey's Philharmagic, The Many Adventures of Winnie the Pooh, Magic Carpets of Aladdin, Tiki Room, Pirates of the Caribbean, Jungle Cruise, Country Bear Jamboree, Haunted Mansion (she was a little scared at 1st, then thought it was silly but HATED it when the ride went backward), wanted to ride Splash but wasn't tall enough, The Indy Cars, Buzz Lightyears Spaceranger Spin, meeting the characters in the Judges Tent, Mickey and Minnie's house, the Barnstormer, and the WDW railroad. We didn't do the Pooh play area, and Donald's boat was being repaired. We actually spent 3 days at the MK (not all day) becasue there was so much for her to do. She loved the parade, Spectromagic and the Dole Whip's.
EPCOT-Turtle Talk with Crush, the Living Seas, the Coral Reef (we had dinner and sat by the tank) all the Imagination pavilion, the Mexico ride, PSB breakfast, the American Adventure show and singers, all the performers, especially the Jamminators and the Chinese acrobats, trying different food, the Kidcot stops, begged to ride Test Track but again too short. She thought Spaceship Earth was dark and boring, except when it went backwards, then she was terrified.
AK-looking at the carvings on the Tree of Life, Festival of the Lion King (Loved it) Affection Section, Kilimanjaro Safari, and the Rainforest Cafe (she liked it more than Chef Mickey's or PSB) Didn't have time to see the Dinoland USA because it was pouring both days.
MGM-The Little Mermaid, Playhouse Disney, MuppetVision 3-D, Beauty and the Beast Live on Stage, Fantasmic.
Even a 3 year old will have plenty to do. Other than the really fast, high rides he'll probably enjoy most of it. Have fun!