How do I sync my iPhone eight to my Apple Watch Series 3

Set up your  Watch -

https://support.apple.com/en-us/HT204505

ASC - Late 2012 Mac mini, macOS 10.14.6; Watch, series 5 GPS/cellular, watchOS 6.2.8; iPad Air 2, ipadOS 13.7; iPhone 8+, iOS 13.7; TV 4th Gen, tvOS 13.4.8;  HomePods, audioOS 13.4.8 and Airport Express w/ Airplay 2