How can I setup FlexUnit 4 to run in a Flex 3 environment?
You can do two things. First, the SDK 3.2 code should work fine for a Flex 3 application. It doesn't contain SDK 3.2, it is just linked against it but there shouldn't be any API changes that would cause you grief.
However, if you do want to build it yourself (and you only need the SDK) the easiest way is actually to build it with FlexBuilder. You'll want to get the project from GitHub (see Getting Started Part 1: Obtaining FlexUnit code (Windows Users)).
After you have downloaded the FlexUnit 4 project and the added projects that you desire (CIListener, UIListener, etc.).
Choose to import an existing project.
Browse to the location where you downloaded FlexUnit4, in this case, FlexUnit 4 is the only project in the directory. Normally, all the projects in the directory are available for import at this point.
Because this is a library project, you should see the .swc file in the specified output directory, in this case it's FlexUnit4/bin.
It should be ready to go. 3.2 SDK is the earliest we recommend using, and if you have a chance to use 3.5, we highly recommend it.