– HD Media – Silverlight 3 now supports hardware graphics acceleration – enabling both video and graphics compositing to be offloaded onto a GPU. This can dramatically lower CPU usage on a computer, and enables HD video to be played on older low end machines. You can now deliver and play 1080p HD video experiences over the web.
– Immersive Graphics – The new GPU acceleration capabilities of Silverlight 3 enable even richer and more immersive graphic experiences.
– Out of Browser Support – Silverlight 3 enables applications to run outside the browser and taken offline. Users can safely install web applications on their computers, and create persistent shortcuts to them on the desktop, start menu and taskbar (this is supported on both Windows and the Mac).
– Application Development – Silverlight 3 includes a ton of new application development features.
– Visual Studio 2008 Tools for Silverlight 3
– Expression Studio 3
Here you can read more about it.