Adding Custom editor buttons in wordpress

Add these below code into functions.php file:

/* Custom editor buttons */
function enable_more_buttons($buttons) {
  $buttons[] = 'hr';
  $buttons[] = 'sub';
  $buttons[] = 'sup';
  $buttons[] = 'fontselect';
  $buttons[] = 'fontsizeselect';

  return $buttons;
}
add_filter("mce_buttons_3", "enable_more_buttons");