Hi Don,
Sorry that you cannot import Sony A7lll files into Lightroom Classic CC 7.3.1 and 7.4.
The Sony A7lll .ARW files are supported in Lightroom Classic CC(version 7.3 and above) and the application should be able to import them.
Are you getting any specific error message? Also, have you used the Camera software to convert/process the images before importing them into Lightroom? Please update to the latest version of Lightroom Classic CC 7.5 from the Creative Cloud desktop app and let us know if you're able to import the images?
Still facing the issue after updating to the latest Lightroom Classic CC version? Refer the list of follow up troubleshooting steps mentioned in this article: Troubleshoot Adobe Camera Raw Compatibility Photoshop Lightroom
Edit: Moved discussion to more relevant community: Lightroom Classic CC — The desktop-focused app
Thanks,
Akash