Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 219)

Merging Multiple Exposures with a RAW Image Recorded
on the Card
You can select a RAW image recorded on the card as the first single exposure. The
selected RAW image will remain intact.
1.
Select [Select image for multi. expo.].
2.
Select the first image.
Turn the < > dial to select the first image, then press < >.
Select [ OK ].
The file number of the selected image will be displayed at the bottom of
the screen.
3.
Take the picture.
When you select the first image, the number of remaining exposures as
set with [
No. of exposures ] will decrease by 1.
219