Try:
include("./../../header.inc");
THe ./ refers to the current directory that you're in.
each ../ refers to up one level.
So:
./../ == ../
./../../ == up two levels from where you are.
You can also try just ../../ if my first attempt doesn't work.
Personally, in all my scripts, I always set up a config file with my most basic global information. In there, if I have multiple directories, I set up an array that holds my directory and path to it. Something like:
<?php
$dir = array(
'root' => $_SERVER['DOCUMENT_ROOT'].'/',
'inc' => $_SERVER['DOCUMENT_ROOT'].'/inc/');
include($dir['inc'].'header.inc');
?>
Hope that helps you some.
~Brett