Sorry, I had thought that all apple products had an app store of some kind. It would seem that the 3rd gen doesn’t though. Sorry. Does it have a web browser? Might be able to use Jellyfin’s web interface.

If you have , or have access to, a Jellyfin server, there is a Jellyfin client called Infuse (paid app). From there, you can do pretty much whatever you want, on the seas or on high street.

I believe a Plex client is also available.

Beyond that, I’m afraid I don’t know much about AppleTVs. Sorry.