Hello, Collin! Welcome back to the Disney Parks Moms Panel.
You are correct in that, after you have completed your first three FastPass+ selections for the day, you may make additional selections - one at a time - by visiting a FastPass+ Kiosk within the same Theme Park as the attractions you wish to select. After you use that additional FastPass+ selection, you may re-visit the FastPass+ Kiosk to select another. You can repeat this process until the Theme Park closes. There are not currently any restrictions on how many times you are able to keep selecting this additional FastPass+ selection at the FastPass+ Kiosk. It is, however, subject to attraction availability. As the day and evening goes on, there are less options available to you. I believe my family's record is currently four FastPass+ selections in one day - it was a late night and we were exhausted but we had a blast!
You are able to make your first three FastPass+ selections 30 days in advance (60 days if staying at a Walt Disney World Resort Hotel). I typically schedule these selections to start around mid-morning. We get to the Theme Park right when it opens, when the wait times are very minimal. Once the crowds pick up it is time to use our FastPass+ selections to reduce our wait times. By booking them earlier, we then have more time to add our additional selections.
Wishing you and your family a magical vacation, Collin!
~Rachel