Even if you still have an Xcode 3.2.6 installer rotting in your archive, you won’t be able to install it on OS X Lion or Mountain Lion. The installer just refuses to install.
That’s a problem if you still need to compile your plugins for R12 or R13, because those releases’ project files are not quite compatible with Xcode 4.4 (and the binaries compiled with Xcode 4.4 don’t work in R12 and R13, either).
Therefore, many developers still keep an installation of OS X (Snow) Leopard on a second Mac or a secondary partition.
Well, it’s not a solution for everybody. But if you’re lucky and still have a working installation of Xcode 3.2.6 on an older OS X version, you can simply copy the whole Developer folder over to OS X (Mountain) Lion. Yes, it’s that easy. It works.
A Tumblr user suggested a way to actually run the Xcode 3.2.6 installer on 10.7 or 10.8: