Yes, the Canon EOS R3 is compatible with the Transcend 700S V90 64GB.
Canon EOS R3 Slot 2 accepts SD UHS-II. Transcend 700S writes at 180 MB/s (V90 class) -- clears 4K 29.97fps ALL-I (requires V60) and all standard SD-recordable modes. Full HD 239.76fps ALL-I slow-motion requires 200 MB/s or higher (Video Speed Class V90 or higher) -- a raw floor, not satisfied by V90 class alone -- and this card's real 180 MB/s write speed falls short by 20 MB/s.
Limitation
Does not meet Canon R3's Full HD 239.76fps slow-motion mode (requires a genuine 200 MB/s sustained write, not just V90 certification) -- this card's real speed is 180 MB/s. Works for 4K 29.97fps ALL-I and all standard SD-slot modes.
“[200 MB/s or higher] Video Speed Class V90 or higher”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)