So I was trying to provision a 3rd gen apple tv today after updating Apple Configurator and found that configurator 2.5 only supports tvOS 10 or higher. I can't find any documentation saying configurator was dropping support for 3rd gen apple tv's.
I talked to an Apple Engineer about this yesterday, we ran into the same issue. He said Apple has heard from customers upset about the lack of 3rd Gen support with Apple Configurator 2.5 and they are going to patch it, but he unfortunately did not have a time frame for the release. His only suggest was to find an older Mac device and use older Configurator version.