You don't have to book 3 say for example if you want to use 2 in one park, then hop to a 2nd park.
After you tap your magic band in the FP line for the 2nd FP, you can go on the
MDE app and book the 3rd in the 2nd park. This will be our plan this coming October. We have booked 2 for the morning at Epcot, then planning on eating lunch and grazing around the world, then trying to book a FP for HS while we are in line for the 2nd FP. CAN'T WAIT!

ENJOY!