The upgrade from Aperture 2 to Aperture 3 is not free. You need to buy Aperture 3 from the App Store.
Before upgrading take a few precautions: Make sure you have a current backup of your Aperture library and of the Aperture application - just in case. Fix any problems with your Aperture library before upgrading, by repairing or rebuilding the library while you are still using Aperture 2, see the release notes (http://support.apple.com/kb/TS2518 - the paragraph "A note on upgrading your Aperture library" at the bottom of the document.)
might hold out for the mythical Aperture 4 / X lol
There are quite a few issues with Aperture 2 on Mavericks - the loop not working, occasional crashes. Aperture 2 has no longer been supported on MacOS X since Lion. If you will wait for Aperture 4, be sure to keep frequent backups of your Aperture library, just in case.