How to use nth-child in CSS to select all elements after a certain one?

Yes you can do it using :nth-child(n+4)

In your case:

@media(max-width:768px) {
   .list li:nth-child(n+4) {
      display:none;
   }
}

You can see this fiddle : http://jsfiddle.net/wgLCH/2/