Inserting code between indexed content

Hello,

I need to add some code between the content thumbnails (Advertisement code to be more specific). I need to place it after the first 3 tiles. Here’s a SCREENSHOT for you so you can get the idea easily.

Below is the ad code that I need to insert:

<!-- Composite Start -->
<div id="M152751ScriptRootC53236">
    <div id="M152751PreloadC53236">
        Loading...
    </div>
    <script>
                (function(){
            var D=new Date(),d=document,b='body',ce='createElement',ac='appendChild',st='style',ds='display',n='none',gi='getElementById';
            var i=d[ce]('iframe');i[st][ds]=n;d[gi]("M152751ScriptRootC53236")[ac](i);try{var iw=i.contentWindow.document;iw.open();iw.writeln("<ht"+"ml><bo"+"dy></bo"+"dy></ht"+"ml>");iw.close();var c=iw[b];}
            catch(e){var iw=d;var c=d[gi]("M152751ScriptRootC53236");}var dv=iw[ce]('div');dv.id="MG_ID";dv[st][ds]=n;dv.innerHTML=53236;c[ac](dv);
            var s=iw[ce]('script');s.async='async';s.defer='defer';s.charset='utf-8';s.src="//jsc.mgid.com/z/o/zonlinemovies.com.53236.js?t="+D.getYear()+D.getMonth()+D.getDate()+D.getHours();c[ac](s);})();
    </script>
</div>
<!-- Composite End -->