Try it Yourself - jQuery sel

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

<h1>This is a heading in body</h1>

<p>The first paragraph in body.</p>
<p>The second paragraph in body (and the 3rd child element in body).</p>

<div style="border:1px solid;">
  <span>This is a span element in div</span>
  <p>The first paragraph in div.</p>
  <p>The second paragraph in another div (and the 3rd child element in this div).</p>
  <p>The last paragraph in div.</p>
</div><br>

<div style="border:1px solid;">
  <p>The first paragraph in another div.</p>
  <p>The second paragraph in another div.</p>
  <p>The last paragraph in another div (and the 3rd child element in this div).</p>
</div>

<p>The last paragraph in body.</p>

</body>
</html>