I tried it with a .htaccess file wich contains;
AddType application/x-httpd-php .php3
but this doesn't work...

When I do it like this and type http://www.domain.index.php3 it asks me if I want to download it...

if I remove the htaccess file it works but the url with /? doesn't so php3 doesn't work correctly i think (i have php4 installed)