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' )
));
}