It is quite easy to use the class files in the ActionScript 3.0 in the development of the Flex 3.0 applications. Even the .swf files can be used to load the flex appliaction on the net.

http://livedocs.adobe.com/flex/3/html/help.html?content=compilers_30.html.

The pakage of the file can be kept in the library files and can be re-used to make the applications.  The better option to design a Flex application is by designing the application in Flash and then exporting the class files to be used in the main mxml application.