BioWare have said though that other platforms and interfaces will be announced in the coming months and you will still be able to gain success in the single player game without having to engage in the Galaxy at War feature.
The multiplayer mode is part of a new feature called “Galaxy at War” which BioWare has fully yet to explain and reveal its full purpose within the new Mass Effect 3 game. This means that multiple users on the same system have to buy their own Online Pass to join the Galaxy at War and play co-op. So I didnt have enough war assets to get to the best ending. “Players can choose from a variety of classes and races, form an elite Special Forces squad, and combine their weapons, powers and abilities to devastating effect as they fight together to liberate key territories from enemy control.” The pass in Mass Effect 3 will be locked to the user account, similar to Mass Effect 2’s Cerberus Network code. I just finished Mass Effect 3, though I didnt play the multiplayer and my war readiness was therefore at only 50. Definitely cant wait to see how Chiefs re-emergence will effect the story. Very interesting layering of halo lore in the story so far. Mass Effect: Legendary Edition is slated to hit shelves. However the new multiplayer gaming mode does tie into your Mass Effect 3 single-player game providing you with an alternative method of achieving success in the games main storyline. Shadow is most likely either a Forerunner or an Ancient Human. Mass Effect: Legendary Edition rebalances Mass Effect 3's Galactic Readiness, making decisions from Mass Effect 1 and 2 carry more weight. Mass Effect 3 is all about fighting a huge war for the sake of the fate of the whole galaxy - which is a pretty difficult thing to portray and allow players to keep track of in a choice-based RPG. As the entire Galaxy prepares for war against the Reapers in Mass Effect 3, Shepard can influence the success or failure of the effort based on. Higher scores increase rates of survival.
Galactic Readiness in Mass Effect 3 is a score determined by many factors, especially Shepard's decisions.
The announcement was officially posted on the BioWare forums and has unveiled that the new 4-player co-op missions take place outside of the single-player game, and aren’t affected by your Mass Effect 1 or 2 games saves. Mass Effect 3: How to Increase Galactic Readiness.