Hi Billie,
Welcome back to planDisney! We are so excited that you are utilizing us to help plan your upcoming vacation. I hope we are helping you make the trip as magical as possible and that with all your questions answered you can leave your worries behind when arrive at the Most Magical Place on Earth!
Theme park reservations are no longer required for date-based tickets at Walt Disney World. When you purchased your 3-day park hopper ticket, you selected the dates you would be visiting and have essentially reserved entrance for those dates. The theme parks will stop selling tickets for those dates if they get close to capacity. You do not need to worry about being able to enter whichever theme park you choose for the day. All four Walt Disney World theme parks are open 365 days a year. Theme park closures do not exist except under the rare circumstances of a natural disaster or event outside of Disney's control.
I do recommend having a plan ahead of time for which theme park you would like to enter each day so that you can maximize your time at each! If you only have 3 days to experience all 4 parks, I would set aside one of those full days for
Magic Kingdom. Magic Kingdom has the most rides and experiences of any of the parks and it is hard to see everything in just one day. Since you are staying at the Polynesian Resort, you have to stop at
Kona Island right before you get on the monorail for the best Caramel Machiatto in all of Disney to get your fuel for a busy Magic Kingdom day!
Happy Planning
Jenna