Code igniter 3 load controller in library

In Code igniter 2.x if a controller function was needed from a library it could be loaded

$this->CI->load->library('../controllers/class');

However in Code iginiter 3 I am getting an error message

Unable to locate the specified class: Session.php

Has anyone else had this issue to deal with. We are in the midst of an upgrade and I am trying to minimize the amount of code to be rewritten.