Hi, I made an swf application with Flex 3 Builder using mxml files. Now i've got to deliver the application to a designer in my division so I would like to know, if he wants to modify the application, does he have to use Flex Builder as well? Adobe CS3 wouldn't be enough?
I'm asking if he can modify the design as you do in flex builder in the design tab (drag drop component ...)
Well, to answer your subject question, Flex Builder is not needed to edit the MXML or ActionScript code that makes up a Flex Application.
You can use notepad--or any text editor, and re-compile it against the command line SDK. There are also alternatives, FDT comes to mind as one. There is also a few Visual Studio plugins.
None of that addresses your real concern. As far as I know, there are no tools in CS3 [or 4] that will give you "Designer-style" access to a Flex Builder project.
It sounds like you want Flash Catalyst; however it is still in beta, and has limited support for the Flex 3 component architecture.