Hey Rafael!
Could you have her wear a thin t-shirt and shorts under the dress? That way if she gets hot it will be as easy as pulling the Princess dress off for a little bit so that she can cool down.
Or, if her Princess dress is too awkward with something underneath, you could bring a t-shirt and shorts with you to the Princess dinner and Party in a backpack.
The Park does have lockers available in case you needed to take off the Princess dress and didn't want to drag it around with you.
Your 3 year old will probably be so tickled at seeing other little girls dressed up like her that she won't want to remove her dress! I doubt that dressing up for these events would be too much for her and in the evening when the sun goes down a lot of that Florida heat goes with it so she should be much cooler.
I hope you have a wonderful, Magical, Princess Pixie Dust kissed trip!