Apple Watch Series 3

Yes.

Read the footnotes on this webpage

https://support.apple.com/en-us/HT205547

iMessage via our Apple Watches is how my Wife and I exchange short messages. We have GPS only Apple Watches.

I have also exchanged some SMS texts with an Andorid user or 2 (SMS/MMS needs the iPhone and you need to be logged into iMessage on your iPhone).

Yes as long as you have your phone with you.