My Apple Watch series 3 won’t accept my passcode.

Hi

As your Apple Watch cannot be unlocked via the normal process, then follow these steps to erase your locked Apple Watch and to re-pair it to your iPhone:

• Connect the Apple Watch to its charging cable, connected to power.
• Whilst keeping the Apple Watch on the charger and connected to power:
  • Press and hold the side button until you see the Power Off slider.
  • Press firmly on Power Off (do not try to slide it - just press down more firmly than a normal tap), release your finger and then tap on Erase all content and settings and confirm when prompted.
• Re-pair the Apple Watch using the Watch app on your iPhone, choosing to Restore from Backup if offered the option.

More information:

Set up your Apple Watch - Apple Support

If you forgot the passcode for your Apple Watch - Apple Support

Check that you are pressing and holding down only the Side button:

A quick look at Apple Watch

If your Apple Watch restarts when you press only the Side button (the lower of the two buttons):

A quick look at Apple Watch

Then that may be consistent with an issue with the operation of the Digital Crown, which your Apple Watch may believe is also being held down at the same time, causing it to force restart.

If you are using a case, bumper or any other kind of protective accessory on your Apple Watch, then it may help to remove it.

Operation of the Digital Crown can also be affected by substances like dust or lotion. It may help to clean it by following the steps here:

Use the Digital Crown on your Apple Watch - Apple Support

If the issue persists, then your Apple Watch may require hardware service. I suggest that you contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider to arrange for your Apple Watch to be checked and, if required, serviced:

• US: Apple Watch Service Answer Center - Apple Support
• Elsewhere: Apple - Support