Hi,
You can find a detailed chart of the new features in Version 3.0.3 here:
http://www.notation.com/WhatsNew.php
There's a specific link right under the title to another page that shows you the changes since Version 3.0.2.
Multiple operating system support, new Notation Software Synth engine, new ASIO support, new Export to Wav option and more.
AND it's a free update for owners of Version 3.
Hope that helps.
David