How to connect my Apple Watch Series 3 into my iPhone 11?

Series 1 to 3

If you have iOS16 or later on the iphone, series 1 to 3 Watches are difficult to pair because of relatively low storage but…

Ensure the iphone is on iOS16.4 or later.

Erase the Watch completely. Put your watch on its charger and keep it there until you complete these steps. (select the watch OS version) Erase Apple Watch – Apple Support (UK) 

When pairing do not try to restore the backup, pair as a New Watch Set up your Apple Watch – Apple Support (UK)

Select the appropriate watchOS from the dropdown menu in this guide --> Set up and pair your Apple Watch with iPhone – Apple Support (UK)