Can I still use my Apple Watch three series if I leave my cell phone at home
Can I still use my three series Apple Watch if I forget my phone and leave it at home
Apple Watch Series 3, watchOS 6
Question marked as
If it is the Series 3 cellular + GPS and you have connected the cellular to it then yes you will have full function.
If it is the GPS model or the Cellular + GPS without connected cellular, then you will only have communications when you are connected to a known WiFi
See Use your Apple Watch without your iPhone nearby