Well this morning I jumped back into Flash CS3. I was pretty shocked about the lack of code hinting for custom ActionScript 3 libraries, I am addicted to Flex Builder now. Also I was confronted by the seeming lack of support for using Flex Builder for ActionScript 3 Flash CS3 editing. It looks like you can use virtual files and folders to fix this problem. Here is how to do it:
1. Open Flex Builder 3 (Beta 1)
2. Create a new ActionScript Project, any project name will do!
3. Delete the generated ActionScript base class (Here is where things get virtual).
4. File > New > File or File > New > Folder
5. Select Advanced on the Dialog, Check “Link to file in the file system”
6. Select Browse and select your “Document Class” AS3 class file.
7. Press “Finish”

If you map all your dependent libraries into this project virtually, you will get code hinting for custom classes in Flex Builder for a Flash CS3 Project. The key is to use the ActionScript project to map to all the files and folders you need, including the class paths within your Flash CS3 application. Assuming everything is there, you will get code hinting for all custom classes, properties, and events.
With projects in Flex Builder (Eclipse), it doesn’t matter where the files are located. Use virtual files and folders to bring files located all over together in one project. This will simplify development and allow you to organize projects easily regardless if the project is Flex, AS3, or Flash CS3.
I am using this for PV3D development and having code hinting for all these custom classes is invaluable. Shoot even organize imports and refactoring works!
Cheers,
Ted