title: How to select 1st, 4th, 7th element and so on?
body: <p>How could I select the 1st, 4th, 7th elements and so on ?! also how to select the 3rd, 6th, 9th and so on?</p>
  
  <p>the pattern is selecting and element then leaving two and selecting the third. I know I should use <code>:nth-child</code>, but I don't know how exactly.</p>