Canon EOS-1 DX Mark III Advanced User's Manual page 382

AF area selection method
You can set the method for switching AF area/AF method selection modes.
[ ] →M-Fn button
After you press the <
area/AF method selection mode.
[ ] →Main Dial
After you press the <
method selection mode.
Note
When [ →Main Dial] is set, use <
> button, pressing the <
> button, turning the <
> to move the AF point horizontally.
382
> button will change the AF
> dial will change the AF area/AF