I do think fallout New Vegas will be the true fallout 3. It's going to have NCR, Real BoS, Ceasers legion, Powder Gangs and locations that were going to be in Van Buren. Some of the details may have changed because it's been 10 years and owned by another company. The Devs working on New Vegas worked on fallout one and two. I have said many times on here (Kinda as a joke) "New Vegas is code for Van Buren ".
Fallout 3 is a great game but to me it had nothing to do with the first to fallouts not really a sequel to fallout two. To me it's like fallout tactics and New Vegas a Side game but still canon.
But the game does say fallout three on it so it is the 3rd game. Can't really change 4 million copies of the game to make old fallout vans happy. Still to me New Vegas is going to be the true sequel. I may change my mind if some how the game is crap but I don't think that is going to happen. It's going to be alot like the first two but made with, 3D and graphics and so on.