I have bad news and good news for you...
Because of the age of AA3.0, Adobe have taken the activation servers for this off line so you can no longer use your existing CD and serial number.
The good news though is that Adobe have provided a download of AA 3 on which you can use a generic serial number which they also provide. You can get details of this from this FAQ: Audition 3 and the activation service I suggest you scroll down to the last few posts which will give you the latest details.
One last point...you'll also find that the download site provides an upgrade to version 3.01. I suggest you also install this--it came out shortly after the original version and fixes a few bugs.
Good luck and happy mixing!
Bob