sub sub sub menu (three levels)

I have this pages menu :

a
-aa
–aaa
–aab
–aac
-ab
-ac

On the page aa, I want to show the wp_list_pages aaa, aab & aac, If I’m on the page aaa, I want to show aaa, aab & aac (nothing else). How can I achieve this? For now, I have this code (and it’s only working for the second level)

<?php
global $wp_query;
$post = $wp_query->post;
$ancestors = get_post_ancestors($post);
if( empty($post->post_parent) ) {
$parent = $post->ID;
} else {
$parent = end($ancestors);
}
if(wp_list_pages(“title_li=&child_of=$parent&echo=0” )) { ?>

<div id=”secondary-nav” role=”navigation” class=”menu”>

<?php wp_list_pages(“title_li=&child_of=$parent&depth=1” ); ?>

</div><!– #secondary-nav –>

Thanls for your help!