The only way to do this without editing the eCart Object is to add another Set Session Value server behavior to the confirm page that will get the first three characters of the post code.
On the server behaviors panel, click the plus button and select eCart -> General -> Set Session Value. Select "Any form Post" for the trigger, set the name to something like "First3PostCode". For the value, click the lightning bolt and select the shipping_postcode form element. this will add code similar to:
<?php echo ((isset($_POST["shipping_postcode"]))?$_POST["shipping_postcode"]:""); ?>
you will need to modify this to use the substr function:
<?php echo ((isset($_POST["shipping_postcode"]))?substr($_POST["shipping_postcode"],0,3):""); ?>
Now when creating the rule in the eCart object, instead of using the "eCartCheckoutForm_shipping_postcode" session variable, use the "First3PostCode" session variable.