Sub- and Subsubmenu problem

I’m trying to make a website with main navigation and a sub- and subsubmenu.

So far I get the main navigation en both sub- and subsubnavigation visible, but when I select a subsub-item, the sub-items disappear…

I tried all kind of different things, but doing something wrong.

Here’s my code so far;

<?php
  if($post->post_parent){

    $children = wp_list_pages('title_li=&child_of='.$post->post_parent.'&echo=0');

  } else {

    $children = wp_list_pages('title_li=&child_of='.$post->ID.'&echo=0');

  }

  if ($children) {

    echo "<ul id=\"accordeon\">";
    echo 	$children;
    echo "</ul>";
  }

?>

The test page with the submenu is available on the following link; http://www.graphixs.nl/vrenken/wordpress/?page_id=8