I’ve recently talked to a few people who’ve experienced issues trying to use Flex SWCs (created in Flex Builder) in Flash CS3, so I thought I’d post what I know on the topic.

For starters, you can’t have any Flex dependencies. This is AS3 and Flash territory-only, friends.

Second, CS3 only supports SWCs that have components (display objects) you can drag to the stage. You’re out of luck with utility classes.

But even components won’t show up automatically. To see them in CS3 you need to specify a manifest file. This lets CS3 know which classes in your SWC can be instantiated as components.

For the uninitiated, you can go here for an example:
http://code.google.com/p/cimlogbook/source/browse/trunk/manifest.xml

In Eclipse, open the Properties for your Flex Library project and select Flex Library Compiler, then specify your manifest file under Compiler options.

Once you’ve built your SWC, drop it in the CS3/en/Configuration/Components folder and it’ll show up in the Components panel. Then you should be good to go!