I have no way to test this, so proceed or ignore at your own risk, but there have been reports that if you have access to a Mac that is running Mavericks, and log in to the App Store with your Apple ID, you can purchase Numbers 3.1 on that machine, and then when you log in to the App Store on your Mountain Lion machine, it will offer to download the older version, since the new one isn't compatible with that operating system.
If you do choose to be a guinea pig for this, it would be nice if you posted back and let us know if it worked.