Looks like we've been somewhat trolled by Xbox 360 World; the magazine's latest cover showcased 'Mass Effect 3 multiplayer'. However, it seems it's not confirmed as we thought.
A user on the BioWare forums said they purchased the magazine and within the edition it said the Mass Effect 3 multiplayer part on their cover was based on rumors:
Well, if you believe the rumours – which we certainly do – ME3 will have MP. According to various sources, there will be a batch of 3 player co-op missions that run parallel to the main story, with each player assuming the role of a single squad mate. It seems likely that this will happen, and it'll form part of EA's Online Pass system. However devs have been known to ditch MP during development in the past, if it doesn't work…
Another poster said on the cover's multiplayer headline for the game:
Despite having 'Multiplayer!' written on the front cover, there is only a 2" by 1" box on one of the pages discussing MP, and as you can see from previous post it is nothing we haven't already heard.
Looks like we're going to have to wait a little longer for EA and BioWare to unveil a multiplayer component for Mass Effect 3.