Canon EOS-1D X Mark III Advanced User's Manual page 264

Manual Exposure Recording
You can manually set the shutter speed, aperture value, and ISO speed for movie recording.
1.
Set the Live View shooting/Movie shooting switch to <
2.
Set the recording mode to <
3.
Set the ISO speed.
Press the <
selection.
Press the < > button.
Turn the < > or <
>.
> button, then turn the <
> dial to set it.
264
>.
> or < > dial for