When will preview in Catalina support the Canon 1Dx iii CR3 files?

There is no present Camera Raw support in Catalina for the Canon EOS 1D X Mk III camera. As we are fellow users, we have no inside information about where Canon and Apple are in sharing that data, testing it, or scheduling its inclusion in a future release of macOS.

The preceding link is a living document so when 10.15.5 arrives, check to see if the requested camera support has been added.

None. Don't even know what a 1Dx III is. Certainly, not an Apple product.

FYI: These are user-only forums. This is not Apple Support. if you need to contact Apple Support, then contact product and tech support:  Contacting Apple for support and service including international calling numbers.

These forums only provide help with Apple products. Thank you for your understanding and support.