I'm guessing you mean the iPhone 3G, since the iTouch 3G and 3GS have exactly the same hardware. Anyway, you need to open the .ipa file in Winrar, extract the .app directory, copy it to you "mobile applications" directory (should be in your itunes media directory), and drag and drop it into the applications tab on itunes. It should install fine when you sync your phone.
Keep in mind you might have a few performance issues, especially if your device is jailbroken. I haven't tested it on my old 3G, but I know it's been somewhat sluggish when playing some of the more processor-intensive games.