No, the Canon EOS R3 is not compatible with the Lexar Professional CFexpress Type A Gold 256GB.
Canon EOS R3 Slot 1 accepts only CFexpress Type B. Lexar Professional CFexpress Type A Gold is a Type A card — physically incompatible with the R3's Type B slot.
What works instead
See all compatible camera for the Canon EOS R3 →“When recording movies, use a CFexpress card that supports VPG400”
Last verified:
Confidence: inferred