<div class="razdel" id="first_item_list">
                            <ul class ="categorylist">
                            <?php 
                                foreach($catalog as $item)
                                {
                                echo '<li id="'.$item['id'].'"> <a class="item" href="'.base_url().'administration/showchild/'.$item['id'].'">'.$item['title'].'</a> </li>';
                                }
                            ?>
                           </ul>
                         </div> <!-- first_item_list -->
                         
                        <div class="razdel" id="second_item_list">
                            
                        </div> <!-- second_item_list -->
                        
                        <div class="razdel" id="thrid_item_list">
                        
                        </div>

$(document).ready(function(){
    $(".item").click(function(e){
             var el = $(this);
             e.preventDefault();
             if(el.closest("#first_item_list"))
             {

                $.ajax({ 
                    url: el.attr("href"),
                    success: function(data)
                    {
				      $("#second_item_list").html(data);
                    }
                 });//$.ajax
             }  
            else
              if(el.closest("#second_item_list"))
                {
                  e.preventDefault();
                     $.ajax({ 
                        url: el.attr("href"),
                        success: function(data)
                                  {
              				      $("#thrid_item_list").html(data);
                                  }
                     });//$.ajax
                 }  
              
        });//$(".item").click(
  }); //$(document).ready(