You won't be able to re activate Audition 3 even if you could find a copy of AA2 since Adobe have discontinued the activation web site for all their CS 2 software that included Audition.. However all is not lost since they have provided a special version with a serial number for AA3 owners that doesn't need to be activated.
Error: Activation Server Unavailable | CS2, Acrobat 7, Audition 3