It's five years overdue at the LEAST! We've had to deal with Unleashed, The secret rings, The black knight, and FOUR yet they can't make a Sonic Adventure 3?! Heroes wasn't even as bad as people let it up to be, at least it played closer to the Adventure formula than today's excuse!
Ever since Sonic Team went through a massive restructuring, all of the new devs just want to focus on the retarded gameplay set forth by the Storybook saga and Unleashed. If Adventure 3 were announced, it would probably be by a split group.
I don't think we need one. We have plenty that can be considered Adventure 3. Sonic Heroes, 06, Unleashed, Colors, and Generations. I think were perfectly fine with out having a direct Sonic Adventure 3 becuase lets face it if they were to make it it would end up like all the other Sonic games now (except the ones I listed above) and we don't want to tarnish the Adventure series with a bad game. Its fine with two.
None of those games above save maybe Sonic Heroes can be considered a true Adventure. Their gameplay are so different from the two Adventure games.
Instead of going back to the Adventure style gameplay, I rather they continue focus on the new Unleashed style gameplay and making that better. After Sonic Generations, I'm convinced this style can work. 

If they do go back to the Adventure style, then they better stick with it rather than they changing to a completely different game style years from now.

Not sure if that made sense, but it seems Sega is confused on what direction this franchise should take, which needs to change immediately. >_<