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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>jQuery nth-last-child(3n) demo</title>
  <style>
    body { font-family: sans-serif; padding: 16px; }
    ul { list-style: none; padding: 0; max-width: 220px; }
    li { padding: 8px 12px; margin: 4px 0; background: #f1f5f9; border-radius: 6px; }
    li.mark { background: #fef3c7; border: 1px solid #f59e0b; }
  </style>
</head>
<body>
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
    <li>Item 4</li>
    <li>Item 5</li>
    <li>Item 6</li>
    <li>Item 7</li>
    <li>Item 8</li>
    <li>Item 9</li>
  </ul>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("li:nth-last-child(3n)").addClass("mark");
  </script>
</body>
</html>

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jQuery li:nth-last-child(3n) — every third from the end (Try It)