Operation Anchorage (January 2009)
The Pitt (March 2009)
Broken steel (May 2009
Point Lookout (June 2009)
Mother ship Zeta (August 2009)
The release date for Fallout New Vegas (October 2010)
Dead money (December 2010)
So in comparison Fallout three has their add ons drop every two to three months.
Might be off on the dates a little, but as We now know going with Obsidian was a mistake and Bethesda should have kept their Fallout 3 developers on New Vegas.
Unless We may see a Fallout 4 with their (Bethesda) new game engine being used on Elder scrolls 5.