Apple Watch series 3

Start by saying why you cannot? What is stopping you? What is happening when you try?

EDIT: Just realised, your Watch most likely is on watchOS 6 which requires iPhone compatible with iOS 13 software and your iPhone 6 is not.

Return your Apple Watch or upgrade your iPhone to newer model compatible with iOS 13 .

Watch Series 3 needs an iPhone 6s or later running iOS 13 or later.

Bring Watch Series 3 back for a refund, you have 14 days if you bought it from an Apple Store or Online.

If you bought it from 3rd party store, check their return policy.

Or you buy a newer iPhone model (iPhone 6s or later) running iOS 13 or later to pair with the Watch.