need some help here :-)

Hi
im making some changes to my new forum design , its an SMF forum, so im putting google adsense into
i have used this code in BoardIndex.template.php

			echo '
			   <table cellspacing="0" cellpadding="3" border="0" align="center" width="95%" class="tborder">
    <tr>
            <td align="center">
				<script type="text/javascript"><!--
google_ad_client = "pub-0803858518022926";
/* 728x90, created 8/30/09 */
google_ad_slot = "6690476404";
google_ad_width = 728;
google_ad_height = 90;
//-->
</script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
		     </td>
      </tr>
      </table>';
	  

Code (markup):

But you know that google adsense allow just 3 banners on 1 page
So my question is: how can i modify the code so its just be shown just 3 times and not after every board
Hope my question is clear
Thanks