Hi Kathryn,
Thank you so much for visiting the Disney Parks Moms Panel with your question. We on the panel love helping guests to plan magical vacations!
Being able to go from one theme park to the next on any given day is a great option. Sometimes, my family likes to start a day in one theme park, and then head back to our resort hotel for some relaxing pool time in the middle of the afternoon. By evening, we are ready to head back to a theme park for an evening meal and fireworks show, often at a different park than the one at which we started the day.
In order to go from one park to another in one day, you do need to add the park hopper option to your
Walt Disney World tickets. You cannot use two separate days' worth of admission to enter two parks on one day. So to do your plan of Magic Kingdom on day one, Disney's Hollywood Studios and Epcot on day two, and Disney's Animal Kingdom on day three, you would need to have a three-day ticket with the park hopper option.
I hope that this information is helpful for you, Kathryn! Please do let us know if you have any further questions.