Canon EOS-1D X Mark III Advanced User's Manual page 260

Aperture-Priority AE
< > recording mode enables you to set your preferred aperture value for movies. ISO
speed and shutter speed are set automatically to suit the brightness and obtain standard
exposure.
1.
Set the Live View shooting/Movie shooting switch to <
2.
Set the recording mode to <
Press the <
selection.
>.
> button, then turn the <
260
>.
> or < > dial for