In addition to that excellent advice, series 3 Watches are difficult to pair because of relatively low storage but…
Ensure if the iphone is on iOS16 then it is on iOS16.4 or later.
Erase the Watch completely. Put your watch on its charger and keep it there until you complete these steps. (select the watch OS version) Erase Apple Watch – Apple Support (UK)
When pairing do not try to restore the backup, pair as a New Watch Set up your Apple Watch – Apple Support (UK)
Ii is important that the previous owner removed it from their Apple account as part of the reset.