Technicaly, there is, so far, no Sonic Adventure 3 because each of those games past SA2 has had something to make it stand out.  Sonic Heroes had more platforming in it thanks to the flying element with the teams, Sonic 06 had the tekekenisis from Silver, and Secret Rings was a spin-off.  So for you people who think that the Adventure seeries is still going, you're dumb!