Create a .php3 file (with the same file name) for all your .php files that contains the following:
<?php
$loc_st = eregi_replace(".php3", ".php",$PHP_SELF);
header("Location: $loc_st");
?>
This will redirect the page back to it's .php counterpart. Don't place this in a .php file, though. I'm sure there is a better way involving configuration of your web-server but this is all I came up with.
-Ben