I keep trying to update my Apple TV but it is never successful. 3rd gen works with most apps but will not update.

If your Apple TV (3rd generation) is on Apple TV Software 7.9, then it is already on the latest version for this model, and there are no further updates at this time.

Update your Apple TV - Apple Support

Version 7.0.3 is from 2015, so that unit hasn’t seen an update in 7 years.

Try reinstalling the system software externally, through a computer. This would get the latest version. That may work when incremental updates through the system itself do not complete.

Restore your Apple TV - Apple Support