I came across an issue with Flex builder 3 today - the JVM (and therefore the whole builder app) crashed when switching to design view. It's a known bug from back in the Beta, but it was "non-reproducible" (damn I hate hearing that).
A helpful soul on the Flex list at House of Fusion suggested some JVM tuning and it seems to be the ticket.
With FB3 closed (i.e. not running), edit
C:\Program Files\Adobe\Flex Builder 3\FlexBuilder.ini
and make the following changes:
Try higher settings of you still have grief. If you never had these problems, great; remember this in case they pop up.