How to connect a Apple Watch Series 3 to iPhone 7?

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

If you are having issues, please explain what those are in as much detail as possible.

Also see here: https://www.igeeksblog.com/apple-watch-not-pairing-with-iphone/

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Include your product (device) name and specs such as processor speed, memory, and storage capacity. Please do not include your Serial Number, IMEI, MEID, or other personal information.

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ASC - Late 2012 Mac mini, macOS 10.14.6; Watch, series 5 GPS/cellular, watchOS 6.2.8; iPad Air 2, ipadOS 13.6.1; iPhone 8+, iOS 13.6.1; TV 4th Gen, tvOS 13.4.8;  HomePods, audioOS 13.4.8 and Airport Express w/ Airplay 2