How to make 3 boxes like this screenshot has?

Your question is far too broad; you need to look at the CSS and HTML source and theme that site is based on to find out how that layout is done. Look at other free WP themes to find a similar layout.

And learn how to use Firebug with Firefox, or use the developer tools in Chrome or Safari or IE to see the CSS and HTML of that site and the site you’re trying to develop.