title: addClass every nth
body: <p>I have a list of elements that I want to style in 3 different ways.</p>
  
  <p>I want every 3rd list item to have the same class throughout the whole list.</p>
  
  <p>For example:</p>
  
  <pre><code>&lt;li class="A"&gt;Some Content&lt;/li&gt;
  &lt;li class="B"&gt;Some Content&lt;/li&gt;
  &lt;li class="C"&gt;Some Content&lt;/li&gt;
  &lt;li class="A"&gt;Some Content&lt;/li&gt;
  &lt;li class="B"&gt;Some Content&lt;/li&gt;
  &lt;li class="C"&gt;Some Content&lt;/li&gt;
  &lt;li class="A"&gt;Some Content&lt;/li&gt;
  &lt;li class="B"&gt;Some Content&lt;/li&gt;
  &lt;li class="C"&gt;Some Content&lt;/li&gt;
  </code></pre>
  
  <p>I can do 2 with :odd/even, but how to do it with 3?</p>