Если запрос правильный. Попробуйте это
$this->db->select('position.po_name AS position, activity.ac_name AS activity , COUNT(register.po_id) AS regis')
->from('register')
->join('activity', 'register.ac_id = activity.ac_id','INNER')
->join('position ', 'register.po_id = position.po_id','INNER')
->where('register.regis_status', 2)
->where('activity.ac_id', $ac_id)
//more where ->where()
->group_by('register.ac_id,register.po_id');