add_role.php

<?php

function action_pmpro_after_change_membership_level( $level_id, $user_id )
{ 
	$user = get_userdata($user_id);
 	$role_levels = array(1,2,3);
	
	if(in_array($level_id, $role_levels))
	{
		$user->add_role( 'subscriber' );
	}
}
         
// add the action 
add_action( 'pmpro_after_change_membership_level', 'action_pmpro_after_change_membership_level', 1, 2 );