I'm hoping that another build is in the works real quick for FlexBuilder 3. The current beta is really flaky. Just switching between views in eclipse occasionally makes all controls on the screen go blank until they are reactivated. They will all come back if you move the panels or exercise the buttons, but it means that you have to move something around on your eclipse toolbar to make them show up again.
Also, all of the interesting work in the design view regarding constraint based layouts appears to have reverted back to Beta 1 or FB2 (I'm not sure which). If you wrote anything in beta 2 that used the new column and row constraints, it stills works in code, but you can't see it in action in design view. Unfortunately, working with these new types of constraints in code view is a painful exercise.
Unfortunately, I've become so confident in these last few betas that I didn't bother to save my installer for the beta 2 (mac plugin version if anybody is feeling generous). Doh!