It is not possible to use the Flex 3 SDK with Flex Builder 2.
One of the new features of Flex Builder 3 is the ability to use
different SDKs - something Flex Builder 2 lacks. If you haven't
tried Flex Builder 3 because you thought you'd not be able to make
Flex 2 applications - you have nothing to worry about. When you
create a Flex 3 project in Flex Builder 3, you can specify that you
want to use the Flex 2 SDK.