Stackoverflow - Applying class to every 3rd list item

http://stackoverflow.com/q/7950193/918414

HTML

<div class="list">1</div>
<div class="list">2</div>
<div class="list">3</div>
<div class="list">1</div>
<div class="list">2</div>
<div class="list">3</div>

















































































































































































































<script>
  var _gaq = _gaq || [];
  _gaq.push(['_setAccount', 'UA-23915674-6']);
  _gaq.push(['_trackPageview']);

  (function() {
    var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
    ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
    var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
  })();
    
var headerUri = 'http://stackoverflow.com/q/7950193/918414',
    headerCaption = 'Applying class to every 3rd list item';
document.body.insertAdjacentHTML( 
    'afterBegin',
      '<a href="' + headerUri + '" '
    + 'target="_top" '
    + 'onmouseover="this.style.opacity=\'.95\'" '
    + 'onmouseout="this.style.opacity=\'1\'" '
    + 'style="'
        + 'background-color: black;'
        + 'background-image: linear-gradient( top, rgba( 255, 255, 255, .3), rgba( 255, 255, 255, 0) );'
        + 'background-image: -webkit-linear-gradient( top, rgba( 255, 255, 255, .3), rgba( 255, 255, 255, 0) );'
        + 'background-image: -moz-linear-gradient( top, rgba( 255, 255, 255, .3), rgba( 255, 255, 255, 0) );'
        + 'background-image: -o-linear-gradient( top, rgba( 255, 255, 255, .3), rgba( 255, 255, 255, 0) );'
        + 'background-image: -ms-linear-gradient( top, rgba( 255, 255, 255, .3), rgba( 255, 255, 255, 0) );'
        + 'border: 1px solid black;'
        + 'border-radius: 2px;'
        + 'color: white;'
        + 'display: block;'
        + 'font: normal 15px/26px Helvetica, Verdana, Tahoma;'
        + 'height: 26px;'
        + 'margin: -8px 0 8px -8px;'
       ...

CSS

.list{
    color:#aaa;    
} 
.grey{
  color:#ff0000;
}

JavaScript

$('.list:nth-child(3n)').addClass('grey');