BioWare isn't talking about any possible multiplayer features for the upcoming Mass Effect 3. "We're exploring multiplayer in all of our games and all of our franchises," BioWare co-founder Ray Muzyka told Eurogamer at E3 last week, not exactly deflecting the ongoing rumor.
Eurogamer reports that sources familiar with the game's development claim that Mass Effect 3 will feature online multiplayer support, through a separate "standalone" co-op mode that allows four players to play against an onslaught of enemies. The mode is supposedly similar to Gears of War's Horde offering, and will not "compromise" the single-player campaign. Developed by BioShock Montreal, the multiplayer component could be be released as an entirely separate game, the rumor further details.
Given BioWare's desire to continue the Mass Effect franchise after the trilogy is done, we wouldn't be surprised to see a standalone multiplayer spin-off. Of course, we'll have to wait and see if this rumor plays out.