^It was called Sonic World Adventure. But I still think it is possible that SEGA makes a Sonic Adventure 3. I mean if it took them that long to create Sonic 4 than it is possible. And besides, did some other games also take that long to make the next number of the game after other releases, for example Contra. It did take them a lot to make number 4.
SEGA was just piggybacking off Capcoms nostalgia cannon success with Megaman and to a lesser extent Bionic Commando, they failed but that's not the point. The reason why it's unlikely to be Sonic Adventure 3 or why Sonic Adventure 3 is likely to ever exist is because not many people really give a shit about Sonic's failed attempt into 3D. Sonic Adventure 3 wouldn't have nearly as big an impact to the fans as Sonic 4 did so really there's no point in it.