ethan.com on "Add Third Menu Bar"

i would like to add a third level menu on wordpress, this is the code i've added to the function.php to create the secondary menu, but when i'm trying to repeat the same code with a third menu, i get an error.

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

Help?