Lore in Mass Effect 3 covers the basic aspects of the Universe that are touched in the game. Previous wars, long-time held grudges between Races, or any important event that is mentioned during the game but isn't exactly done by Commander Shepard and the Normandy crew can be found here.
Mass Effect 3 Lore
The lore of Mass Effect 3 is set during the year of 2186, where the Milky Way Galaxy is under invasion of an advanced synthetic-organic race known as the Reapers. Feared for their ritualistic eradication of organic civilizations every 50,000 years, the story begins with the Reapers effortlessly overwhelming planet Earth. In response to the invasion, the Systems Alliance orders the protagonist, Commander Shepard, to travel to Mars in search for counter-measures against this unprecedented force. Upon arriving Mars, Shepard learns from Liara of a Prothean superweapon capable of obliterating the Reapers. He manages to obtain the schematics of the superweapon from a mysterious figure known as the Illusive Man, who in the process, reveals the Cerberus' scheme to control the Reapers. With the schematics at hand, the Alliance begins the construction of the superweapon now coined the "Crucible", and sends for Commander Shepard on a quest to gather allies across the galaxy.
Shepard's quest begins with his mission on the planet Palaven, where he rescues the Turians from the planet's orbiting moon, Menae. In response, the Turians offer Shepard their support, under the condition that the Krogans must aid them in defending against the Menae. When confronted by Shepard, the Krogans insisted that the Turians must first cure "Genophage", a biological weapon they had used to infect the Krogans. This prompted Shepard and the leader of Krogans to travel to Sur'Kesh, the home world of a race known as Salarians. There they meet a Salarian scientist who formulates a cure from a fertile Krogan female known as Eve, and plans to utilize the tower known as "Shroud" to diffuse it across Tunchaka, where the Krogans inhabit. As Shepard revisits Tunchaka, the Salarian leader contacts him and reveals that the Shroud has been sabotaged, and that the Salarians will fully support the removal of enemies on the Shroud should he wish to proceed with the plan to cure Genophage. Shepard is then presented with two options: request the Salarians to reclaim the tower, resulting in the Salarian scientist's death, or secretly halt the mission to cleanse Genophage, misleading the Krogans into believing they have been cured.
Regardless, after the events that take place on Tunchaka, Cerberus forces will attempt to take over the Citadel, although they do not succeed. Commander Shepard is then presented with the next task, where the Quarians propose a deal to the Alliance - support them in their quest to reclaim their home planet, Rannoch, from Geth Troops, in exchange for their allegiance. Shepard then proceeds to sabotage a Geth Dreadnought after recuing a captive Geth ally, and disables the Reaper control signal. Shepard then proceeds to destroy one of the Reaper base on Rannoch, achieving a window of opportunity for the Quarians to strike. Unfortunately, the rescued Geth unit decides to upgrade the rest of the Geths with Reaper technology by sacrificing itself, which overwhelms the Quarian forces, and bestows free will, as well as sentience, upon all Geths. Shepard is again presented with two options - either negotiate a ceasefire to the support from both races, or pick one race to support, which would completely obliterate the other.
Following the conclusion of the Quarians' arc, an Asari Councilor summons Commander Shepard to the Citadel. She discloses the news of an ancient Prothean artifact on Thessia, the native planet of the Asari, which may help identify a component of the Crucible, known as the Catalyst. Shepard goes on to discover a Prothean VI on Thessia called "Vendetta", however the recurring villain, Kai Leng, steals it as Thessia falls under Reaper invasion. Shepard tracks Leng down and ends up at the Sanctuary facility at Horizon, a human colony world where the Cerberus has been secretly developing technologies to control the Reapers. Shepard obtains a tracker that leads him to the Illusive Man's headquarters, and initiates a full on attack at the Cerberus space station with the Alliance forces. Shepard eventually defeats and kills Kai Leng in battle, and upon retrieving the Vendetta, learns that the Catalyst is the Citadel itself. At this point, however, the Citadel has fallen under the Reaper's control, who, after successfully indoctrinating the Illusive Man, ensured he had escaped before the Alliance's assault.
The Alliance, along with its allies, launch an full on assault on the Reapers as a final attempt to reclaim Earth, and activate the Crucible by merging it with the Citadel. Shepard enters the Citadel through a transportation beam in London. After his battling the Illusive Man, Shepard finds himself being elevated up to the top of the Citadel instead, and comes face-to-face with a childlike synthetic being that self proclaims to be the Catalyst and the creator of Reapers. The final fate of Commander Shepard, his Normandy-SR2 squad, and the entire galaxy, rests in the player's choice in dealing with the Catalyst, as well as their final EMS score. It is revealed in a post-credits scene that a man known as "Stargazer" tells the legendary tale of the Shepard to a young boy, implying that his legacy lives on long beyond the ME3 timeline.