I want to unpair an I watch series 3

if it was me then I would try

How to pair your Apple Watch with a new iPhone - Apple Support

scroll down to "If you don't have your old iPhone or you erased it"

To unpair and erase watch and remove Activation Lock

From a web browser,sign in to iCloud.com with your Apple ID.

Go to Find iPhone.

Click All Devices at the top of the screen.

Click your Apple Watch.

Click Erase [device]. Select Next until the device is erased.

Click Remove from Account.