I curious to know if there is now a good work around in ThumbsPlus to create thumbnail images of the more recent camera RAW formats, specifically .CR3.
I don't think there is. ThumbsPlus offers two plug-ins for loading raw files, and both are based on DCRAW which hasn't been updated since 2016.
I think I'd try two things:
1) Create a filetype definition for the .cr3 format, tell TP to load it through the Digicam or Digiraw plug-in (try both) and in the options of that plug-in, tell TP to use the .jpg thumbnail that is embedded in raw files. With luck, one of the plug-ins might manage to retrieve the thumbnail even if it doesn't fully support the .cr3 format.
2) Apparently the Libraw raw development toolkit comes with a program named dcraw_emu.exe that emulates DCRAW but that is up to date in terms of raw formats supported. When you look in your TP10 program folder, there's a BIN sub-folder that contains the original DCRAW.EXE. You could rename that e.g. to DCRAW-old.exe and then copy DCRAW_EMU.EXE into that folder and rename it to DCRAW.EXE. That way, you will have replaced the old dcraw with a new one that should function very similarly to the old one, except with support for new raw formats. Then launch TP and see if you can get it to load your .cr3 files (and any other raw files). If this works, you should not have to specify in the plugin options that the embedded thumbnail should be used (though for the purpose of thumbnailing, it is quite a bit faster than developing the actual raw data).