In an interview with role-playing forum community Mass Effect Universe, BioWare producer Jesse Houston clears up the enduring rumors that Mass Effect 3 will have a multiplayer mode. Long story short: no multiplayer for ME3.
The closing question is the Q&A of interest:
Q: [T]here's been a persistant rumour, perpetuated by the fan community itself, that ME3 will feature in some shape or form, multiplayer.
Now Jesse, the number one concern from fans is that a seemingly 'mediocre' throw-in of multiplayer will detract significantly from what is and has been for a while, a really really good singleplayer experience, and responses from BioWare have been somewhat cryptic and vague at best..... could you do us all a favour and put this rumour to rest with a definitive answer? Pretty please?
A: As for the Multiplayer rumours - They are just rumours. We're really focused on delivering the best single player story experience that we possibly can and one that can tie up all the loose ends and finish the trilogy with the love and devotion that it deserves!
In short, it looks as though Mass Effect 3 will remain a single-player only experience.
Thanks to Redditor UnicornPuppy for the info.