What are differences and similarities among css , css3 and w3css ?

Answer 1

Css is the generic name.
Css 3 is the versionned name, as before were Css 1 and Css 2.

W3css is nothing...

... but it could be:

W3C is the accronyme of the World Wide Web Consortium.
W3s.css is the name of a Bootstrap-like CSS standalone framework provided by w3schools.com ( they provide a JS framework too, but it isn't required ).

Answer 2

It could also be the name of a seperate spreadsheet. In that case it would actually be W3.css, not W3css.