Add New Buttons to the WordPress Editor

functions.php

//* Enable hidden buttons to third row of post editor
function my_custom_mce_buttons_3( $buttons ) {	
	/**
	 * Enable core button(s) that are disabled by default
	 */

	$buttons[] = 'fontselect';
	$buttons[] = 'fontsizeselect';
	$buttons[] = 'wp_page';
	$buttons[] = 'superscript';
	$buttons[] = 'subscript';
	$buttons[] = 'underline';

	return $buttons;
}
add_filter( 'mce_buttons_3', 'my_custom_mce_buttons_3' );