The first downloadable content is the "Horde Command Pack", which was released on November 1, 2011. In addition to new character and weapon skins, it provides three new maps for use in Versus and Horde mode and three new fortifications that the players can build.
The second downloadable content is called "Versus Booster Map Pack". It was released on November 24, 2011 and is available for free. The pack contains five multiplayer maps which includes Blood Drive, Azura, and Rustlung, which were originally made available through the Horde Command Pack; and Bullet Marsh and Clocktower, which will be new to Gears of War 3.
The third downloadable content is "RAAM's Shadow". The expansion was released on December 13, 2011. Included in this DLC is four new multiplayer characters, a new weapon set named "Chocolate", new achievements, and over 3 hours of singleplayer gameplay. The story takes place before the first Gears of War, set in Sera post-emergence day. Zeta Squad is enlisted to evacuate Ilima City and protect the citizens against a Locust Kryll storm. The player will be in the role of General RAAM, the main antagonist from the first Gears of War. Zeta is led by Michael Barrick (from “Gears of War” comics fame) and composed of returning characters such as Lt. Minh Young Kim, from the first Gears of War and Tai Kaliso from Gears of War 2, and also includes a new female character, Alicia Valera.
The fourth downloadable content is "Fenix Rising", scheduled for release on January 17, 2012. The DLC will include five new multiplayer maps and will also introduce four new character skins and a new system that allows players to rank up through level 100 multiple times.
Though from what I can tell, the Versus Booster Pack doesn't count as one of the four season pass packs, so there should be another one after Fenix Rising.