Rumors are that Mass Effect 3 will feature a four-player co-op mode. Rather than being an element of the single-player game, it would be like a Horde-mode option where players battle waves of enemies together and may come as a part of the main game or as a standalone product.
“We haven’t talked at all about multiplayer in the Mass Effect franchise, but we’re exploring multiplayer in all of our games and all of our franchises,” said BioWare co-founder Ray Muzyka at E3. “It’s always an idea we’re interested in. The short answer is we have no comment on that. But we’re interested in the idea of multiplayer.”
Source: Eurogamer