(September 2020 update: CR3 support has been added to the built-in plugin in current Opus beta versions. You may not need Microsoft's extension at all now.)
For Windows 10 users, Microsoft offer a Raw Image Extension codec which adds support for various raw camera image formats to both File Explorer and Directory Opus.
While Opus comes with the Raw Camera plugin, you may find Microsoft's codec supports some camera models better (or vice versa).
You can get the codec from the Microsoft Store. At the time of writing, it requires Windows 10 1903, and was just updated to support the CR3 files some newer Canon cameras produce.
Edit: It turns out the CR3 update is only for Windows Insider build users at the moment, but presumably for everyone in time.
Edit 2: It is now available to everyone.
Thanks to @ForzaAndy for the tip!