If it were a brand new camera, then no it will not be supported in macOS High Sierra.
As this is a slightly upgraded model then it may be as long as Sony have not changed the way it writes the RAW file information.
Apple only ever updates its Camera RAW Database with its most recent macOS which now will be macOS Monterey 12.0.1.
They never make the Database backwards compatible.
Read the article below, note it has not been updated since October 2018, about the time macOS Mojave was introduced.
Digital camera RAW formats supported by iOS 11 and macOS High Sierra - Apple Support
The Sony Alpha ILCE-7R IIIa does not even make it in to the supported list for macOS Monterey,
Digital camera RAW formats supported by iOS 15, iPadOS 15, and macOS Monterey - Apple Support (HK)
But as it is not listed in any of the Digital camera RAW formats supported by macOS whatever it is difficult to tell.
If you can find somewhere online that provides downloadable Sony Alpha ILCE-7R IIIa RAW files see if High Sierra can
open them. Otherwise contact Sony, but knowing their support I wouldn't hold my breath.