According to Game Informer magazine (via NeoGAF), the game will continue from the events of Mass Effect 2 DLC Arrival, seeing Shepard standing trial back on Earth.
The prologue will see the Reapers attack the planet, leading to Shepard escaping to the Normandy and then recruiting allies to fight off the galaxy-wide invasion.
The Illusive Man will apparently play a larger role in the adventure, wherein Cerberus is now out to kill Shepard instead of helping him.
Confirmed squadmates include Liara, Ashley / Kaiden, Garrus and a new character named James Sanders. While Wrex, Mordin, Legion and Anderson will potentially return, they are all believed to be non-controllable side characters.
The story's conclusion will offer numerous endings based on player choices and which characters are recruited over the course of the game.
Role-playing elements will be expanded from those seen in the second game, with larger skill trees, powers that evolve over time and weapons that can be upgraded with modifications.
All classes in the game can now use any weapon, but will be limited by the amount they can carry at once. The Soldier class will be able to bring everything to the battlefield, for example.
Finally, BioWare has denied early reports that the game will feature multiplayer, but will allegedly consider such a feature for 'future games and spinoffs in the series'.
The firm has previously discussed redesigning the HUD and enemy types, and has confirmed that Black Swan composer Clint Mansell will work on the game's soundtrack.
Mass Effect 3 will be available on Xbox 360, PlayStation 3 and PC later this year.
Watch the announcement trailer for Mass Effect 3 below: