load Controller in Controller, CI3

hi
I can't load controller in another controller in CI3, but in CI2 i do this without problem!!!
for test plz create MY_Controller in core and load a test library or model or etc
then in welcome controller or each controller you want, load another controller like my codes then see errors, i cant know why this doesn't work in CI3 plz help me

and other question why CI2 benchmark is very better than CI3? it isn't bad? do you recommended me for use CI2?

"please sorry if you found any mistake in my english"

---

MY_Controller:

PHP Code:

class MY_Controller extends CI_Controller
{
    public function __construct()
    {
        parent::__construct();

        $this->load->library('cart'); // any library or model you like, you can load
    }
} 


and this is my welcome controller

PHP Code:

defined('BASEPATH') OR exit('No direct script access allowed');

class Welcome extends MY_Controller {

    public function index()
    {
        $this->load->library('../controllers/_404');
        $this->_404->index();
    }
} 


and my _404 controller

PHP Code:

defined('BASEPATH') OR exit('No direct script access allowed');

class _404 extends MY_Controller
{
    public function index()
    {
        echo '404';
    }
}