Hello Betsy!
First of all, I sure wish you had more than one day to visit Walt Disney World. However, it's better to have one day at Walt Disney World than no days at all!
If I had the chance to visit only one Disney Park, I would choose
Magic Kingdom hands down! Magic Kingdom has everything a three year old would want from incredibly themed Disney Attractions to MAGICAL Disney Character Meet and Greets.
Now, I bet your three year old really likes Disney movies. Fantasyland in Magic Kingdom will actually bring to life some of the greatest and most beloved Disney movies and characters like Dumbo, Peter Pan, and Winnie the Pooh right before your eyes. Also, there is nothing like walking down Main Street, USA to see Cinderella Castle. In fact, watching
Wishes Nighttime Spectacular fireworks show with Cinderella Castle as the backdrop is my favorite memory of
every Walt Disney World Vacation!
As you can see, you can't go wrong in visiting Magic Kingdom! If you have other questions, please let me know!
Have a MAGICAL day!
Wes