Hi Syreeta!
With a 3 year old, you should be able to do every attraction at the Magic Kingdom, except:
Space Mountain
Splash Mountain
Big Thunder Mountain Railroad
He might be scared by the 3D effects in Mickey's PhilharMagic, but I suggest giving it a try. The movie is awesome and a lot of fun. He also might be scared by the Haunted Mansion. Children that young are sometimes just frightened by the name; it's actually a lot of fun and not scary inside. Also Snow White's Scary Adventures and Stitch's Great Escape may be too intense for your son; trust me when I say you won't be missing much if you skip Stitch's Great Escape!
Be sure to order the free Walt Disney World
planning DVD.
I also recommend picking up the book "The Complete Walt Disney World"
by Julie and Mike Neal. There are a lot of good tour books out there,
but I especially like this for first timers as it is packed with photos
and some good tips; the photos help to get everybody excited for the
trip and give a glimpse of all of the wonderful things to do inside and
outside of the theme parks.
Finally, be sure to stop by Guest Services in Town Square after first entering the park and pick up a "Birthday Button" for your son! Enjoy planning and write back if you have any more questions!