Macbook Pro (2012) with High Sierra 10.13.1. Have Photoshop CS6 & CS 2018. Can't convert CR3 files.

If your camera is indeed the Canon 90D then it takes camera raw 11.4 or newer to open the CR3 files.

macOS High Sierra 10.13 and photoshop cc 2018 should support your 90D if you install Camera Raw 12.4

from here if the Creative Cloud Desktop application doesn't work:

https://helpx.adobe.com/camera-raw/kb/camera-raw-plug-in-installer.html

Just in case your interested you should be able to upgrade your existing MacBook Pro to macOS Mojave 10.14 or macOS Catalina 10.15.

Mojave if you have a lot of 32 bit applications, since 32 bot apps don't work on Catalina.

https://support.apple.com/en-us/HT211683

You may be able to just update your camera raw filter in Adobe Cloud. Scroll to the bottom (Sorry Adobe does not understand/embrace the fundamental UI/UX design principle of alphabetical navigation). Let us know if you missed that as an option because it was at the bottom.

You will need to upgrade to use the newer raw formats. Time to upgrade your equipment and PS. You can try downloading Bridge, I don't know if it will run on your old computer. If you can get Bridge and ACR to work you might be able to open them. If you have PS 2018, you should have CC and be able to do it.