<?php
	//Automatic Submenu
    global $wp_query;
 	//If current page has no parent...
    if( empty($wp_query->post->post_parent) ) 
      {
     echo "";		
     } 
		 
    else 
	//current page has parents
     {
		$me=$wp_query->post->ID;
		$children = wp_list_pages("title_li=&child_of=$me&echo=0"); 
		if ($children) 
		{
			$parent1 = $wp_query->post->post_parent;
			//show menu with current page's siblings
			echo "<ul class='submenus'>";
			wp_list_pages("title_li=&child_of=$parent1&depth=1");
			echo "</ul>";
			
			$parent2 = $wp_query->post->ID;
			//show menu with current page's children
			echo "<ul class='subsubmenu'>";
			wp_list_pages("title_li=&child_of=$parent2&depth=1");
			echo "</ul>";
			
		}
		else
		{
			$parent1 = $wp_query->post->post_parent;
			//show ONLY menu with current page's siblings		
			echo "<ul class='submenus'>";
			wp_list_pages("title_li=&child_of=$parent1&depth=1");
			echo "</ul>";
		}
	}
	?>

This was for a customer. 
Basically it does this:
If a page has no children, it shows ... nothing, 
if a page has a parent, it shows all other children of this parent (siblings) and the current page
if a page has a parent AND children (3rd level), it shows two separate menus, one with the siblings and current page and one with the children of the third level. 
Maybe it helps someone.

3 Level Navigation with Wordpress

wordpress, navigation

PHP