Canon EOS-1D X Mark III Advanced User's Manual page 217

6.
Set [Continue Mult-exp].
Select an option, then press <
With [1 shot only], multiple-exposure shooting will be canceled
automatically after the shooting ends.
With [Continuously], multiple-exposure shooting continues until the
setting in step 2 is set to [Disable].
7.
Shoot the first exposure.
When [On:Func/Ctrl] is set, the captured image will be displayed.
The <
You can check the number of remaining exposures, as displayed at
right in the viewfinder or on the screen (1).
Pressing the <
( ).
> icon blinks.
> button enables you to view the captured image
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