Sorry to gatecrash this thread.
I too am having to convert to Sagely 3.0, I've followed the instructions here and can confirm that mcrypt is installed.
But when I go to SagePay I just get some HTML code displaying in Safari on a Mac, and a 'Malformed' error about currency in Chrome???
So it looks like I've done something wrong.
If you could take a look at my code that would be great.