How to target grandchild of post_parent using wp_list_pages

I have a much larger problem I posted on another forum (still unanswered) which I might be able to solve if I can find a way to target the grandchild (or third level post) of a parent post.

Right now, this code:

$Pages = wp_list_pages('child_of='.($post->post_parent != false ? $post->post_parent : $post->ID).'&title_li=&echo=0')

    echo $Pages;

Will show 2nd and 3rd level pages (bold=visible in submenu).

• Home
• Parent (active page)

• Child
• Grandchild

I need to find a way to preform a str_replaceon the grandchild.

I know how to target the child, but basically need to do the exact same thing for the grandchildren.

$Pages = wp_list_pages('child_of='.($post->post_parent != false ? $post->post_parent : $post->ID).'&title_li=&echo=0')

    $Pages = str_replace($Pages,
						 '<ul id="test">'.$Pages.'</ul>',
						 $Pages);
	echo $Pages;
	unset($Pages);