Hi Annie! My advice is to get well-acquainted with the
Disney World Website. It is your greatest planning resource. It looks like there are some specials running during May. I would begin by going to this link
Disney World Special Offers and put in your dates. It will show you what resorts are available and what the rates will be. All of the Disney resorts are wonderful. They all offer free transportation to all 4 parks, both water parks and Downtown Disney. It depends on your preference of resort price, theme, etc.
You should also start looking at making dining reservations. You can book them online at
Disney World Dining Reservations. Use this link to look at all of the restaurants. Your 3 year old may really enjoy the character dining experiences. I recommend planning your park visits around where your dining reservations for that day are. Example, if you have lunch booked for a restaurant in Epcot, then I would plan to spend that day at Epcot. If you have breakfast at Chef Mickey's (my family's favorite), then you may want to go straight from there (it is located at the Contemporary Resort) and take the monorail into the Magic Kingdom.
One other great thing that Disney offers are
Disney World customized maps . These let you and your family research the attractions ahead of time and it will send you a custom map with all of your favorites highlighted.
Have fun planning and a have a magical trip!