Your Recordsets account for the month but not the year. Try:
<?php
$Month1 = new WA_MySQLi_RS("Month1",$DBConnection);
$Month1->setQuery("SELECT SUM(OrderAmount) AS OrderValue FROM orders WHERE OrderDate >= DATE_FORMAT(CURDATE(), '%Y-%m-01')");
$Month1->execute();
?>
<?php
$Month2 = new WA_MySQLi_RS("Month2",$DBConnection);
$Month2->setQuery("SELECT SUM(OrderAmount) AS OrderValue FROM orders WHERE OrderDate >= DATE_FORMAT(CURDATE(), '%Y-%m-01') - INTERVAL 1 MONTH");
$Month2->execute();
?>
<?php
$Month3 = new WA_MySQLi_RS("Month3",$DBConnection);
$Month3->setQuery("SELECT SUM(OrderAmount) AS OrderValue FROM orders WHERE OrderDate >= DATE_FORMAT(CURDATE(), '%Y-%m-01') - INTERVAL 2 MONTH");
$Month3->execute();
?>