How can I run Warcraft III (a PowerPC app) on OS X Mountain Lion?
Two ideas I can think of.  First, dual boot with OS X 10.6, which still supports Rosetta.  

The other is to obtain a copy of Snow Leopard Server, which had a license which allowed you to run it in a virtual machine.  (The license for normal SL did not allow you to run it in a VM.  Lion and Mountain Lion changed the license to allow it.) Then use Parallels or VMWare Fusion to run it.  However, I don't believe the graphic support is very good with guest OS X machines, so it may not run well. 

Even if you can find a way to install the app, IIRC Warcraft III was a PPC binary for the game as well, so you'd still be stuck.