Alexandria is actually not the codename for Silverlight 3. It is a codename for some technology leveraged by Silverlight 3, but not Silverlight 3 itself.
Here are some important sessions:
What’s New in Silverlight 3
A summary of all new features in Silverlight 3.
http://sessions.visitmix.com/MIX09/T14F
Amazing Business Centric Apps in Silverlight 3
Covers domain services (connecting server data to Silverlight client), the data grid, paging, sorting, filtering, the data form control, validation and more.
http://sessions.visitmix.com/MIX09/T40F
Intro to the Data Form Control
Not a Mix Session but a great deep follow-up to the above session. Goes in depth on the Data Form control, related attributes, etc.
http://silverlight.net/learn/learnvideo.aspx?video=187317