Conditionally add custom body class in WordPress

functions.php

<?php

add_action( 'body_class', 'wpspeak_conditional_body_class');
function wpspeak_conditional_body_class( $classes ) {
  if ( is_page( array( 'page-1', 'page-2', 'page-3' ) ) ) {
		$classes[] = 'my-custom-page';
	}
  return $classes;
}