Canon EOS R3 Advanced User's Manual page 54

2.
Insert the card.
Card 1 (CFexpress card)
Card 2 (SD card)
The card in the rear card slot is [ ] (CFexpress card), and the one in
front of it is [ ] (SD card).
CFexpress card: With the card label facing you, insert the open side of
the card (2) into the card slot. Inserting cards the wrong way may
damage the camera.
The card 1 eject button (gray) pops out.
SD card: With the card label facing you, insert the card into the card
slot until it clicks into place.
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