User blog comment:Commdor/Rumor: ME3 to Feature 4-Player Co-op Missions/@comment-3578615-20110618151847/@comment-1221773-20110618184240

No, the Game Informer article has a section where it's stated that the rumors that a multiplayer spinoff set in the ME universe would be announced instead of ME3 turned out to be false. The section goes to quote Casey Hudson saying "...we've been trying to think of a way that makes sense for people to experience Mass Effect with their friends. We haven't yet come up with a way to do that, so we don't have anything to announce at this time. But, obviously, multiplayer is something we want to do more of in the future as a company."

Hudson in no way confirms multiplayer will be a part of ME3, but he doesn't quash the possibility either. The GI article doesn't give us a definitive answer to this question.