انها محاولة لنفسك - jQuery sel_nthchild

<!DOCTYPE html>
<html>
<head>
<script src="/lib/jquery-1.12.2.min.js"></script>
<script>
$(document).ready(function(){
   $("p:nth-child(3n+2)").css("background-color", "yellow");
});
</script>
</head>
<body>

<p>The p:nth-child(3n+2) selects each 3rd paragraph, starting at the 2nd child element.

<div style="border:1px solid;">
  <span>This is a span element</span>
  <p>The first paragraph (and the second child in div).</p>
  <p>The second paragraph.</p>
  <p>The third paragraph.</p>
  <p>The fourth paragraph.</p>
  <p>The fifth paragraph.</p>
  <p>The sixth paragraph.</p>
  <p>The seventh paragraph.</p>
  <p>The eight paragraph.</p>
</div><br>

</body>
</html>