Chris Priesty of BioWare has revealed some information about the upcoming finale to Commander Shepard's saga on BioWare's Social Network Forums. In his post, he talks about the inclusion of four player co-op multiplayer, and additional ways in which events in the single player game can be influenced by actions outside of it.
In Mass Effect 3's multiplayer mode, players will create their own customised characters and form into four player "Special Forces" squads. Players will be required to work together to liberate areas from enemy control, and success will influence the single player campaign, providing an alternate route to success.
The other new addition is entitled Mass Effect 3: Galaxy at War. The key mechanic is the "Galactic Readiness" level, which is measured by Commander Shepard's ability to manage the upcoming conflict with the invading Reapers. The co-op multiplayer will affect this readiness level, but BioWare has hinted that there will be more ways to affect this aside from playing the single player campaign.
Chris Priestly has indicated that the multiplayer and Galaxy at War are both optional, and the fight against the Reapers can be an entirely solo affair. Mass Effect 3 is currently set for release in North America on March 6, 2012.