Canon EOS R3 Advanced User's Manual page 937

Customize buttons
You can assign frequently used functions to camera buttons that are easy for you to use.
Different functions, for use when shooting still photos or movies, can be assigned to the
same button.
1.
Select [
2.
Select a camera control.
3.
Select a function to assign.
: Customize buttons].
Press < > to set it.
You can configure advanced settings for functions labeled with
[ ] in the lower left of the screen by pressing the <
937
> button.