Actually, if
Microsoft themselves are to be believed, Halo 3 did more in
three months than Halo Reach did in
two years.
http://uk.gamespot.com/news/ms-177-million-360s-sold-6184291
So this means Halo 3 sold 8 times faster than Halo:Reach did across the same period if your Halo:Reach sales are accurate.
Eight times faster. Halo ODST and Reach have both done extremely poorly compared to Halo 3, they have
both caused damage to the Halo brand. There is no reason to believe this series couldn't have expanded beyond Halo 3, and the reason it didn't was because ODST was overpriced, lacked multiplayer and wasn't well reviewed compared to the others.