Australian magazine PC Powerplay is showing off the cover for its next issue on the publication's official site, and it says as clear as you can get that the game features multiplayer.
In big writing, cross the middle of the front page, it says:
MULTIPLAYER COMES TO MASS EFFECT 3: FIGHT ALONGSIDE YOUR FRIENDS AS THE GALAXY GOES TO WAR!
Note this isn't a scan, or a fan site. This image is posted directly on the website of the magazine itself.
If that's not enough, the description of the mag reads:
That's right - Mass Effect 3 is getting multiplayer! We travel to BioWare's office in Edmonton for hands-on time with the series' first foray into online gameplay, and chat with Mass Effect Series Producer and Mass Effect 3 Project Director Casey Hudson about the decision to go multiplayer in the epic battle for the galaxy.
Whether the third game in the series features multiplayer or not has been talked about for months now, the most recent occasion being just last week. As it appears almost certain the game does include the feature—most likely some kind of co-op gaming—it's time to start moving on from questioning whether it has multiplayer and accepting what it will mean.