The story is the 'epic conclusion' to the story started in Mass Effect and continued in Mass Effect 2, as well as the conclusion of Commander Shepard's story. The tone is expected to continue to get darker, yet also bring back some fun and lightness and include a lot more humor.
The player's choices in the previous games are expected to have a much more significant impact on the plot than the choices in Mass Effect had on Mass Effect 2. As Mass Effect 3 is the end of the planned trilogy, the developers are not constrained by the necessity of allowing the story to diverge, yet also continue into the next chapter. This will result in a story that diverges into wildly different conclusions based on the player's actions in the first two chapters.
The story between Mass Effect 2 and Mass Effect 3 is told through DLC for Mass Effect 2. The Lair of the Shadow Broker downloadable content is the first piece of DLC to begin to expand the story in preparation for Mass Effect 3. Arrival is the second and final piece of DLC that expands the story and bridges the gap between Mass Effect 2 and Mass Effect 3.