The Keychron K3 keyboard has two general modes: Mac/IOS and Win/Android.
In the Win/Android mode, the F1-12 keys do not work for me (neither by pressing fn, nor after using the fn+X+L toggle).
The F keys do work in the Mac/IOS mode (using fn).
The disadvantage of the Mac/IOS mode is that the alt and meta keys are swapped (both left & right).
To work around this I remapped them using udev's hwdb.
To get the keycode of the key you want to remap, use evtest. showkey and evtest show different scancodes.