Code:

<style> ul > li { display: none } </style>
<script src="jquery.js"></script>
<script>$(function() {

    $('ul > li:nth-child(3n+1)').css('display','block');

})</script>
<style>

    ul > li:nth-child(3n+1) { display: block } 

</style>

<ul>
 <li>CSS3 select this element.</li>
 <li>jQuery select this element.</li>
 <li></li>
 <li>CSS3 select this element.</li>
 <li>jQuery select this element.</li>
<ul>

Patch:

Index: selector.js
===================================================================
--- selector.js (revision 3472)
+++ selector.js (working copy)
@@ -372,7 +372,7 @@
                                        if ( first == 1 ) {
                                                if ( last == 0 || node.nodeIndex == last )
                                                        add = true;
-                                       } else if ( (node.nodeIndex + last) % first == 0 )
+                                       } else if ( (node.nodeIndex - last) % first == 0 )
                                                add = true;
 
                                        if ( add ^ not )