Welcome to planDisney, Nicole. Thanks for reaching out to us!
It is very likely that your complimentary ticket is not date-based, which means it's not assigned to a specific window of dates. As such, you will have to make a
Theme Park Reservation for the first park you'd like to visit for the day. When you go to make your reservation, you will be prompted to pick a ticket that you're making a reservation for, and you'll select that ticket and follow the prompts. At this time, you'll be able to hop to another park at any point as long as you've tapped into the park for which you made the reservation first. If you picked dates from a calendar when purchasing your 3-day base ticket, they are date-based, and if that's the case, you would not have to make a Theme Park Reservation for the days you're using those tickets for admission.
The good news, Nicolle, is it's pretty rare these days for Theme Park Reservations to be completely booked, especially for all parks in one day. This usually only happens during peak times. That being said, once you've solidified your plans, I would highly recommend that you make your Theme Park Reservation as soon as possible as there is no guarantee of availability. If there's still availability later on, you can modify it to a different park if your plans change.
If you have any other questions,
I'd be very happy to answer them for you! And if you're a
Star Wars fan, you're going to want to check and see if
the new adventures being added to
Star Tours - The Adventures Continue this year are available at this classic attraction at
Disney's Hollywood Studios when you're there!
Always in motion is the future,
Matt