Satisfy 3 conditions in a If/Else statement php page

what basically I need is to satisfy 3 differente conditions in a If/Else statement that require_once a file in a specific folder. My problem is that when my page is loaded my function satisfy only two conditions. I'd like to know if there's a better method to define this function:

      if ($sel_year <= '2016') {

                          if ($sel_month >= '09') {

                            require_once 'tab2.php';

                          if ($sel_month <= '08') {

                            require_once 'tab1.php';

                          }

                          else


                          require_once 'tab3.php';
                      }

Thank you so much for your help