Hi Paula,
When my little girls were 3, their favorite attraction at Epcot was
The Seas with Nemo and Friends Pavilion. In fact, my 6 year old still loves it more than anything else at Epcot. This slow-moving mesmerizing attraction ends in a multilevel aquarium. Your little girl can enjoy looking at clown fish, dolphin, turtles and more. We could spend hours in this building (and have on several trips).
Another fun attraction for preschoolers is the
Imagination Pavilion. After the captivating ride, you end in a room with "magic carpet squares" that she can jump on to make the sounds of the picture she lands on. For example, jump on the elephant and it will make an elephant noise, etc.
Inside the
Mexico Pavilion is a boat ride similar to, "It's a Small World". My toddlers always enjoyed it. Another ride she could go on is
Spaceship Earth although it won't be as interactive for her as the other ones. However, once the ride ends, there is an area with games that she could enjoy.
I recommend that you spend at least a half day at Epcot and then Park Hop back over to the Magic Kingdom.
Hope this helps and have a wonderful vacation,
Kaylene