Codeigniter expert needed

I am already a Codeigniter developer and stuck at a place.

Here is the scenario

include more then two controllers inside the controller function without class extend.

How can I achieve this?

Controller :
class Dashboard extends CI_Controller {

function __construct() {
parent::__construct();
}

public function cron() {
include APPPATH . 'controllers/controlpanel_api/Dashboard.php';
$dashboard = new Dashboard();
$dashboard->function_test1();

include APPPATH . 'controllers/controlpanel_api/Invoice.php';
$invoice = new Invoice();
$invoice->function_test2();

include APPPATH . 'controllers/controlpanel_api/Messages.php';
$msg = new Messages();
$msg->function_test3();
}
}