Last June we took our 3 yr old to WDW for the first time. For the most part things went pretty good.
You can make a schedule of things to do when & where BUT remember to be flexible. A few days we threw out our schedule & just "winged" it.
Magic Kingdom is a must for any 3 yrs old, but really all the parks are wonderful. But since you might only be there 3 or 4 days, I would skip Epcot.
I guess I would mainly concrete on MGM & MK.
At MGM you don't want to miss Disney's Playhouse for your 3 yr old, also Little Mermaid, Muppets 4D. My son also enjoyed the Great Movie Ride & ToT (he is a dare devil).
At Magic Kingdom there is so much to see & do. If your son isn't afraid of characters make sure to hang around Town Square to meet some of the characters
As far as rides, Fantasyland will pretty be your 3 yr old's favorite. I would try to get on Peter Pan, Winnie the Pooh, Small World, Dumbo, Buzz Lightyear. Depending on your child's 'fear' but my child also really enjoyed the Haunted Manison & Splash Mountain.
I would also try to ride in the Monorail. My son REALLY enjoyed that. Asked to ride in the front with the driver. My son thought he was so cool when we got to ride in the front.
If your child isn't scared of characters, I would schedule some charcater breakfasts too. Chef Mickey & Cape May are two pretty good breakfasts.
If you have time you might want to go over to Animal Kingdom, especially if your child love animals. Ride the Safari & try to see the Festival of Lion King (my son was in awe of it)
Most important just go with the flow...and don't try to see everything!
HAVE FUN!