It looks like the ability to read the CR3 format may be available in freshly-compiled distributions of darktable soon, at least for some cameras. It relies on the exiv2 image library, which added support[1] for CR3 in v0.27.4.

That said, there are some notes in the darktable issue[2] that suggest that the darktable rawspeed library also requires some updates[3].

[1] - https://github.com/Exiv2/exiv2/releases/tag/v0.27.4

[2] - https://github.com/darktable-org/darktable/issues/2170

[3] - https://github.com/darktable-org/rawspeed/pull/271

ART, a fork of RawTherapee, works great with the CR3 files from my EOS 90D.

https://bitbucket.org/agriggio/art/wiki/Home

I have used RawTherapee https://www.rawtherapee.com/