Hi,
first of all I would rename the POST var array to something else. It should work with the code above but it is always a better idea not to use php reserved words as variable names.
What about putting the complete post array into the session, something like
$SESSION['array'] = $POST['array'];
You can then access that array from cart_3.php with
$_SESSION['array'][0] and so on ...
or if you use e.g. userdata instead of array
$SESSION['userdata'] = $POST['userdata']; and
$_SESSION['userdata'][0]