At the time the documentation was written, only Flex Builder 2 was available. Agreed, that instructions should include Flex Builder 3 as that is what is available now. We'll raise a bug to get that resolved.
YOu can compile using Flex Builder 3 by changing the compile version in Flex Builder 3 (which uses Flex 2 SDK (hotfix 3, which includes hotfix 2).
To compile using Flex 2 SDK in Flex Builder 3:
1. Right-click your project and select Properties.
2. Select Flex Compiler.
3. In the Flex SDK version pane, select Use a specific SDK and select Flex 2.0.1 Hotfix 3 from the dropdown list.
4. Click Apply and then OK.
5. Compile as you would normally. Now it will use the Flex 2.0.1 (hotfix 3) SDK for that project.
Hope that helps!