[Pablo]

Your stylesheet is called stylesheet3.css, yet (according to your teacher) in your xhtml file your are referencing stylesheet.css. You also include a reference to page_styles.css, but that file is not part of your ePub, as far as I can see.

You can have as many stylesheets as you like in your ePub, and use as many of them in each xhtml file as you want, but you have to include references, in each xhtml file, to the stylesheets you want to apply to that file.

Including a stylesheet in your ePub has no effect until you include a reference to it somewhere. That's the error you see in calibre editor.