Canon EOS-1D X Mark III Advanced User's Manual page 352

Setting the initial Servo AF position
You can manually set the initial Servo AF position when [
] is set to [Initial AF pt set for
1.
Set the AF method to [
2.
Set AF operation to [Servo AF] (
3.
Set the AF point.
/
To move the AF point (1), use <
To return the AF point to the center of the screen, press <
< > or < > button.
: Initial Servo AF pt,
] ( ).
+Tracking] ( ).
).
> or tap the screen.
352
/
> or the