Jquery select multiply of 3 :    Hi in my Ul li i want to select only muliply of 3 other wise it will show alert msg. How i achive this  

    <ul>
          <li><input type="checkbox" />a</li>
          <li><input type="checkbox"/>b</li>
          <li><input type="checkbox"/>c</li>
          <li><input type="checkbox"/>d</li>
          <li><input type="checkbox" />e</li>
          <li><input type="checkbox"/>f</li>
        </ul>