If you are on Windows, I can confirm from what PixelGeniius has told me that PK Sharpener should work in CS3, but I am yet to install it - that is scheduled for tomorrow. If I have pain I'll inform accordingly. Likewise I shall also be trying to install Noise Ninja.
Plug-ins normally need to be re-installed from your original plug-in program disc or download, and serial numbers re-entered in the same way as if you were doing it the first time.