Yes, the Canon EOS R3 is compatible with the Samsung PRO Plus 256GB SD.
Canon EOS R3 Slot 2 accepts SD UHS-I. Samsung Pro Plus is UHS-I V30 — meets 4K IPB modes (requires UHS Speed Class 3) and standard Full HD. Cannot record 4K ALL-I (requires V60), Full HD 120fps (requires V60), or Full HD 239fps slow-motion (requires V90).
Conditions for compatibility
Compatible for 4K 59.94fps IPB and 4K 29.97fps IPB (UHS Speed Class 3 sufficient) and standard Full HD modes. Cannot record 4K 29.97fps ALL-I (requires V60 — this card is V30), Full HD 119fps slow-motion (requires V60), or Full HD 239fps slow-motion (requires V90). Not recommended for 4K ALL-I or slow-motion workflows.
Limitation
UHS-I V30 card. 4K ALL-I and slow-motion Full HD (119fps/239fps) not possible — those modes require V60 or V90. 4K IPB and standard Full HD only.
“ALL-I Video Speed Class V60 or higher IPB (Standard) UHS Speed Class 3 or higher”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)