Three step order form, how to go to next step

In your order.php you will need something like this:

$step = (isset($_POST['step'])) ? $_POST['step'] : 'one';

switch ($step) {
    case 'one':
      // do step one
      break;
    case 'two' :
      // do step two
      break;
    case 'three' :
      // do step three
      break;
}

Your form, of course, has to send the appropriate arguments. I used POST, which seems most appropriate for this kind of thing, but the same would be true for GET.

This makes everything process on your one page which WordPress knows about and that will greatly simplify things.