Can’t load music library on 3rd gen ATV box

Apple TV (3rd generation) is not compatible with Apple Music and its online iCloud Music Library, if that is what you mean.

Apple TV (3rd generation) should be able to access your Home Sharing library with local music files (not from the Apple Music subscription however).

Apple TV (3rd generation) might lose some functionality if it has the dreaded “Activation Failed” error. I expect that this would also impact Home Sharing and AirPlay.

failing to activate

There are a few other threads about that on this forum. Not sure how widespread it is, but we did see a recent influx in such reports.

I would suggest to contact an expert from the Apple Support staff for further assistance. Contact Apple for support and service (phone numbers) or start your support request online (chat/​email/​callback).