Canon EOS-1D X Mark III Advanced User's Manual page 381

Select AF area selec. mode
You can limit the selectable AF area selection modes to suit your shooting preferences.
Select the desired AF area selection mode and press <
[OK] to register the setting. The AF area selection modes are described on
Selection Modes (Viewfinder
Caution
The [ ] mark cannot be removed from [Manual selection:1 pt AF].
Even if you add a checkmark [ ] in [Select AF area selec. mode], some AF area
selection modes are not available with certain lenses.
Shooting).
381
> to add a checkmark [ ]. Select
AF Area