Add third level menu on wordpress

Add a new item to the array like displayed below:

add_action( 'init', 'my_custom_menus' );
  function my_custom_menus() {
     register_nav_menus(
        array(
        'primary-menu' => __( 'Primary Menu' ),
        'secondary-menu' => __( 'Secondary Menu' ),
        'third-menu' => __( 'Third Menu' )
        ));
  }