Porting to 13 would cause a lot of regressions. This has been attempted with other devices in the past and has proved not to be worth it.
You will still get the occasional update on 3rd gen, but it's essentially on life support at this point.
While I'm sure your device works fine, it has been EOL for a very long time now, and it's incredibly insecure no matter what OS you install. I highly recommend getting a newer device if you care about security and privacy.