Flex, for one, is Actionscript 3.
Depends on which Flex version you are using.. Flex 1 for example isn't AS 3.0
Anyway, Flex > Flash. Flex is a dev environment with much better built components then the ones in Flash. So if you want better components and more flexibility then go for Flex. From what I know not all Flex components can be adapted correctly with Flash, mainly because Flex is quite different from Flash and it has tons of classes that do not exist in Flash. There will be situations when you can create flex components that behave well inside Flash but once stuff gets more and more complex the harder will you be able to make Flex components communicate correctly with Flash.