Canon EOS-1D X Mark III Advanced User's Manual page 322

AF Status Indicator (Viewfinder Shooting)
< > appears in the lower right of the viewfinder's field of view as the camera continues
autofocusing while you press the shutter button halfway.
In One-Shot AF mode, the icon also appears while you press the shutter button halfway
after the camera has focused on a subject.
Note
You can change the display position of the AF status indicator to outside the
viewfinder's field of view ( ).
322