CI3 CI_Controller extending

Hi!

I would like to create an Api controller extending the basic CI_Controller.
After that I could extend API_Controller to create separate API controllers like users api etc.

I've just read some solutions for older version but that looks strange for my.
For example adding __autoload function to the config.php.

I tried to put MY_Api.php into application/core folder and it doesn't work.

Thanks for the help!