Canon EOS R3 Advanced User's Manual page 338

Shutter-Priority AE
[ ] recording mode enables you to set your preferred shutter speed for movies. ISO
speed and aperture values are set automatically to suit the brightness and obtain standard
exposure.
1.
Set the recording mode to [
2.
Set the shutter speed (1).
Press the <
[ ].
Set it by looking at the screen as you turn the <
Available shutter speeds vary by frame rate (
].
> button, then turn the <
338
> dial to select
> dial.
).