Hmmm, I have bought and received Aperture 3, and whilst waiting for it to arrive, I downloaded the demo, and used the product key supplied by Apple.
I had Aperture 2, but trashed all the Aperture files and app that I could find, and when installing the Version off of the CD, it never asked for any previous version...
I have a feeling it's another of the misnomer's that Apple throw at you, to kinda make you do the right thing! Like SL upgrade!