Apple Watch compatibility
Is Apple Watch se riesco 3 compatibile with iPhone 12?
Apple Watch Series 3, watchOS 8
Yes it is but it is an old watch with low internal memory. It has been a struggle since iOS16
Ensure the iPhone is on iOS16.2 or later
If the Watch has been recently in use by you, erase the watch in Watch Settings, General, Reset, Erase all contet and settings
Then pair as usual
If no better you may want to unpair then set up without restoring any Watch backup on your phone