IF interested in having a 3rd fob (should one get misplaced), the best time to get the 3rd fob made is prior to when its needed.
Programming a 3rd fob is relatively straight-forward if there's two working fobs on hand.
This video is for the F-150 with push-button ignition.
How To Program A 2018 - 2021 Ford F-150 Key Fob - DIY F150 Remote Programming Pairing - YouTube
Used OEM fobs with new / uncut keys are readily available on flea bay.
Quite a few locksmiths can cut the two-sided keys (for unlocking the door) with their router-type key-cutting machine.
I have the XLT with the key ignition. I now have a 3rd key on hand using a near new OEM key, and following similar procedures for programming the 3rd key.
Edited to add: the best keyword for searching for a compatible OEM key fob is to use the FCC ID number printed in really small letters on the fob. To make it easier to read the really small letters and numbers, a cell phone camera can take a close-up photo, and the photo can be enlarged when viewing.