convert your map page to a php page and use php code to generate the api key:
<?php
switch ((isset($_SERVER["SERVER_NAME"]))?$_SERVER["SERVER_NAME"]:"") {
case "domain1.com":
$key = "key1";
break;
case "domain2.com":
$key = "key2";
break;
case "domain3.com":
$key = "key3";
break;
default:
$key = "key1";
break;
}
?>
<script type="text/javascript" src="http://maps.google.com/maps?file=api&v=2&key=<?php echo $key; ?>">/*wagmp*/</script>