Canon CR3 for CS6

The adobe dng converter 12.4 should work on macOS 10.13 High Sierra and handle the Canon EOS 90D .cr3 files.

http://download.adobe.com/pub/adobe/dng/mac/DNGConverter_12_4.dmg

I had been using CS6 on a Windows Vista machine (that recently died) and for my newer cameras I started converting the RAW files to .DNG format using the Adobe DNG Converter. Took care of my problems. It was something I needed to do anyway in case my heirs ever want to look at the images.

I'm not sure what you mean by "10.3 is the latest OS I can go to."

2009 Mac models would have shipped with OS X 10.5 or 10.6 (released in August 2009) and would generally be upgradable to at least OS X 10.11 El Capitan. Even though El Cap is very much obsolete and unsupported by Apple, it would give you the best bet at software compatibility. OS X upgrades are free...

Be aware that CR2 (older) and CR3 (newer) are generic file extensions for Canon RAW files from nearly all Canon cameras.

Each and every camera model from every camera manufacturer has its own, unique Raw file format. So, the devil is in the details. We need to know which Canon camera model you have.

Adobe foresaw the unnecessary "dance" outlined by @TheDigitalDog in the post above where software vendors have to keep issuing updates for new camera models - and users have to keep updating their software. Adobe created the DNG file format for that very reason. If camera makers included DNG in their cameras, end users would not have a problem with file formats and being forced to wait for updates for their shiny new camera. However, most camera makers have not included the DNG option in their  cameras - they stick to RAW and JPG only.

Using the free Adobe DNG converter, you do have the option to convert all your CR3 files to DNG format and then open  the converted DNG files (not the CR3 files) in PS CS6.

https://helpx.adobe.com/camera-raw/digital-negative.html

The latest version of Camera Raw that works with CS6 is 9.1.1, so although you seem to have been able to install version 12.4, it won't work with CS6, which is most likely still using 9.1.1 (or older).

The workaround is to use the free Adobe DNG converter, which will convert (copies of) your CR3 files to DNG files, that can be opened in 9.1.1/CS6.

See https://helpx.adobe.com/camera-raw/using/adobe-dng-converter.html

Read the instructions carefully. Pay particular attention to the fact that the DNG converter cannot convert individual files.

You have to point it to a folder, and all raw files in the folder will be converted.

Exactly what model of Canon?

See: https://helpx.adobe.com/camera-raw/kb/camera-raw-plug-supported-cameras.html
IF your camera is so new, it isn't listed here, nothing you can do for the time being. 
Every time Canon (or Nikon, or Sony etc) release a new cameras, they do this stupid trick of making yet a new proprietary raw file that everyone outside the company, including Adobe need access to so they can 'hack' the tiny differences to support that new proprietary raw. You the customer get to wait and are forced to use the manufacturers raw converter until this all takes place. It is silly but that's how it has been for decades. Some companines are faster to do this than others, but the facts are, every 3rd party raw converter manufacturer has to go through this silly dance. Complain about this to your camera company.