I haven't experienced this for myself, but apparently as of December 13th 2012 the activation service for Audition 3 and some other products has been disabled because of a security issue. So if you need to uninstall and reinstall Audition 3 you will have to follow the instructions on this page.
It's important to note that the software you may have on a disk won't work any more - you have to use the version on the download page, and the serial number provided with it; no other combination will work.
If there are any other issues with this, please post comments on the appropriate forum.