Need help with pairing my existing watch to my phone
How do I pair existing Apple Watch to my new phone?
Apple Watch Series 3, watchOS 8
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Be aware that you can have multiple watches connected to a single iPhone but not the other way around.
If you followed the procedure in:
How to pair your Apple Watch with a new iPhone - Apple Support
where does it fail?
This article may also help:
If your Apple Watch isn't connected or paired with your iPhone - Apple Support