Applying class to every 3rd list item
<script>
$(document).ready(function() {
    $("#contentlist li:eq(2)").css({marginRight: '0'});
});
</script>

$( '#contentlist li:nth-child(3n)' ).css({marginRight: '0'});

<ul id="contentlist">
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>1</li>
    <li>2</li>
    <li>3</li>
</ul>

$( '#contentlist li:nth-child(3n)' ).css( {marginLeft: '20px'} );

$("#contentlist li:nth-child(3n)").css({marginRight: '0'});