download user guide for apple watch series 3

No, but if you open the Books app on your iPhone and go to the Book Store, you can download the guide. Just do a search for

"Apple Watch" in the Books app.

Apple Watch User Guide - Apple Support

On a computer you can right click on the main page and select "save as pdf".

I did exactly this in the "opera" web browser in windows,in the right click menu there is an option to "save as pdf".