Yes, the Canon EOS R3 is compatible with the Delkin Devices POWER CFexpress Type B 128GB.
Canon EOS R3 Slot 1 accepts CFexpress Type B. Delkin Devices POWER CFexpress Type B 128GB has a manufacturer-rated 805 MB/s sustained write speed (no official VPG certification for this SKU) -- clears every R3 recording mode including 6K RAW and 4K 120fps HFR, which Canon's own support page states requires a card rated over 400 MB/s sustained.
“CFexpress memory card Type B VPG400 support SD memory card Compatible with UHS-II”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)