Canon EOS-1D X Mark III Advanced User's Manual page 350

Eye Detection AF
With the AF method set to [
1.
Select [
2.
Select [Enable].
3.
Aim the camera at the subject.
+Tracking], you can shoot with the subject's eyes in focus.
: Eye Detection AF].
An AF point is displayed around their eye.
You can tap the screen to select an eye for focus. Note that if your
selected eye is not detected, an eye to focus on is selected
automatically.
When multiple eyes are detected, you can use <
for focus.
350
> to select an eye