I’m trying to connect my Apple Watch 3 series

To pair an Apple Watch with an iPhone, see this article: https://support.apple.com/guide/watch/set-up-and-pair-your-apple-watch-with-iphone-apdde4d6f98e/watchos

Is there a particular problem you are having with pairing?

series 3 watch won’t connect to phone. Keeps saying that phone needs updating. But has been updated. Pairs but then Un-pairs.