Update Series 3 watch
Stuck on preparing watchOS 8.1 after a very long download
FWIW, a lot of people have problems updating a series 3 watch due to insufficient memory. In some cases, unpairing the watch, completely erasing it, then pairing and restoring the backup will resolve the issue after an update. (Simply unpairing and pairing again may not work.)
Unpair and erase your Apple Watch - Apple Support
Also, see fireyjack's reply to:
Can't update her series 3 watch. - Apple Community
Apple Watch series 3 updates.. - Apple Community