$current_cat_id = get_query_var('cat');
$child = get_category($current_cat_id);
$level2 = $child->parent;
$child2 = get_category($level2);
$level3 = $child2->parent;
if(is_category(6)){
include "catalog.php";
} else if ($level2 == 6){
include "catalog_level2.php";
} else if ($level3 == 6){
include "catalog_level3.php";
}