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I'm working on this site here: http://new.leicesterymca.co.uk/youth-community/our-work/

On this page you will see I have the 1st level menu at the top, then I have PHP code saying if there is a 2nd level menu then display the 2nd level menu in the blue strip.

But now I need the same for the 3rd level menu (on the left of the page). So Essentially I need the code to see if there are any 3rd level pages, if so then display the 3rd level pages here. The code I have for the second level is:

<?php
  $menu = wp_nav_menu(
      array (
        'theme_location' => 'header-menu',
        'sub_menu' => true,
        'echo' => FALSE,
        'fallback_cb' => '__return_false'
      )
  );
  if ( ! empty ( $menu ) )
  {
    echo '<div class="sub-nav-container-full">
          <div class="container">
          <div class="sub-page-menu">';
    echo $menu;
    echo '</div></div></div>';
  }
?>

So, I'm assuming I can use similar code but I don't know how to aim it at the 3rd level only?

If anyone could help, that would be great! :)

Thanks, Shaun.

EXTRA - I also have this bit of code in my functions file if it helps...

// FUNCTION FOR SETTING UP SUB MENU PAGES

// add hook
add_filter( 'wp_nav_menu_objects', 'my_wp_nav_menu_objects_sub_menu', 10, 2 );

// filter_hook function to react on sub_menu flag
function my_wp_nav_menu_objects_sub_menu( $sorted_menu_items, $args ) {
  if ( isset( $args->sub_menu ) ) {
    $root_id = 0;

// find the current menu item
foreach ( $sorted_menu_items as $menu_item ) {
  if ( $menu_item->current ) {
    // set the root id based on whether the current menu item has a parent or not
    $root_id = ( $menu_item->menu_item_parent ) ? $menu_item->menu_item_parent : $menu_item->ID;
    break;
  }
}

// find the top level parent
if ( ! isset( $args->direct_parent ) ) {
  $prev_root_id = $root_id;
  while ( $prev_root_id != 0 ) {
    foreach ( $sorted_menu_items as $menu_item ) {
      if ( $menu_item->ID == $prev_root_id ) {
        $prev_root_id = $menu_item->menu_item_parent;
        // don't set the root_id to 0 if we've reached the top of the menu
        if ( $prev_root_id != 0 ) $root_id = $menu_item->menu_item_parent;
        break;
      } 
    }
  }
}

$menu_item_parents = array();
foreach ( $sorted_menu_items as $key => $item ) {
  // init menu_item_parents
  if ( $item->ID == $root_id ) $menu_item_parents[] = $item->ID;

  if ( in_array( $item->menu_item_parent, $menu_item_parents ) ) {
    // part of sub-tree: keep!
    $menu_item_parents[] = $item->ID;
  } else if ( ! ( isset( $args->show_parent ) && in_array( $item->ID, $menu_item_parents ) ) ) {
    // not part of sub-tree: away with it!
    unset( $sorted_menu_items[$key] );
  }
}

return $sorted_menu_items;
  } else {
return $sorted_menu_items;
  }
}

It seems your question is similar to this one and I think you can get this to work for you as well !

https://wordpress.stackexchange.com/questions/45161/3-level-deep-navigation-menu-not-showing-all-levels