There are so many things to see and do at each of the theme parks, and with three daughters, you might find yourself pulled in a dozen directions all at once! The first thing I would do is let the girls browse the website at
www.disneyworld.com (for Universal Studios, you will have to go to
www.universalorland.com ) and have them each select something they want to do at each park, and make those three things top priority. This is how I do things with my girls, and at the end of the day, I can at least say to each of them that they got to do something they were looking forward to.
At the Magic Kingdom, in addition to what the girls choose, I would do the classics - Pirates of the Caribbean, the Haunted Mansion, It's a Small World, etc. Those are rides that are most closely associated with Walt Disney World, and my girls enjoy them no matter how many times we ride them. My kids also love Big Thunder Mountain and Splash Mountain.
EPCOT is filled with incredible things, and I have three kids that love the cultural aspect of the park. We spend a lot of time in the World Showcase, watching the performers and enjoying the food. Some other must do's for our girls are Test Track - a huge favorite that my four year old is hoping to ride on our next trip - and the Geosphere right at the beginning.
Have a wonderful vacation!