Hello, Katharine!
What a great way to celebrate a third birthday!
There are so many resorts your family will love, so I'll suggest a few based on what you've mentioned.
Disney's Beach Club Resort is a great resort within walking distance to Epcot; the International Gateway entrance to Epcot is only a few minutes away! In addition, Stormalong Bay is an absolutely fantastic pool your entire family will enjoy.
Another option is
Bay Lake Tower at Disney's Contemporary Resort. Staying near the monorail line and having access to a kitchen / kitchenette is great, too. Even a
walk to Magic Kingdom will only take ten minutes!
You could also book a Royal Room at
Disney's Port Orleans Resort - Riverside. The detail in these rooms are so clever; there are even fiber optic lights in the headboard, simulating a fireworks display!
Finally, you will be impressed with the new
Disney's Art of Animation Resort. The pool area is wonderful, and the food options at the food court are really impressive as well.
To celebrate, try Chef Mickey's at Disney's Contemporary Resort, for a great meal, a birthday cupcake and singing! Pick up a complimentary birthday button at Guest Services, too. A princess makeover at
Bibbidi Bobbidi Boutique would be a highlight of her day, too (followed by a meal with the princesses either at Cinderella's Royal Table or at Akershus Royal Banquet Hall.)
So many possibilities! Send more questions as you narrow down your choices; I'd love to help!