FWIW, a lot of people have problems updating a series 3 watch due to insufficient memory.
To Free-Up Storage
For songs, see:
Remove music from Apple Watch - Apple Support
and for pictures, see the "Limit photo storage on Apple Watch section in:
Choose a photo album and manage storage on Apple Watch - Apple Support
To delete apps on your watch, see:
Delete apps from your Apple Watch - Apple Support
Also, try unpairing and pairing your watch again and restoring from the backup just created in the unpairing process. This deletes all temp and corrupted files and re-indexes the file system. This should result in no loss of data and in some cases can free a lot of storage.
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support
Also see:
If you don’t have enough space to update your Apple Watch - Apple Support
Another option (maybe even better, if possible) is to take to make an appointment at the Genius Bar in an Apple Store near you and let them update it.
Genius Bar Reservation and Apple Support Options - Apple