nth child selector

nth child selector

Hi all,

I have the following and would like to target every 3rd "frame class".  ive tried
  1.             $("#gallery .thumbs a .frame:nth-child(3n)")
but I think 'm not understanding the nested divs correctly.
  1. <div id="gallery"> <span class = "portfolio_heading">
  2.   <p>portfolio/nature</p>
  3.   </span>
  4.   <div class="thumbs"><a href="?page=botanical"><div class = "frame"><img src="images/thumbs/botanical_thumb.jpg" />
  5.     <div class="title">
  6.       <h3>botanical</h3>
  7.     </div></div>
  8.     </a></div>
  9.   <div class="thumbs"><a href="?page=landscape"><div class = "frame"><img src="images/thumbs/landscape_thumb.jpg" />
  10.     <div class="title">
  11.       <h3>landscapes</h3>
  12.     </div></div>
  13.     </a></div>
  14.   <div class="thumbs" style="float:right;"><a href="?page=horses"><div class = "frame"><img src="images/thumbs/horse_thumb.jpg" />
  15.     <div class="title">
  16.       <h3>wild desert horses</h3>
  17.     </div></div>
  18.     </a></div>
  19.   <div class="thumbs"><a href="?page=wildlife"><div class = "frame"><img src="images/thumbs/wildlife_thumb.jpg" />
  20.     <div class="title">
  21.       <h3>wildlife</h3>
  22.     </div></div>
  23.     </a></div>
  24. </div>
Thanks