BioWare revealed today that a pre-release demo for Mass Effect 3 will arrive in January 2012. However, Battlefield 3 owners will get a chance to play it early.
Owners of Battlefield 3 who activate their Online Pass will get early access to the multiplayer side of the demo. In other words, you'll need to either buy a new copy of BF3 or get a used copy and buy the separate Online Pass. The early access will be applied to the EA account you have associated with your Online Pass. The length of this early access period is not clear.
BF3 owners won't get the single-player portion of the demo early, though. Everyone will have to wait until January to try out ME3's campaign. The exact date of the demo's release will be announced in late November.
If you're not interested in BF3 but want the ME3 demo early access, there's another way to get it. However, EA hasn't said what this alternate program is. They'll make that announcement in late November or early December.
Mass Effect 3's multiplayer is a series of four-player co-op missions. Players don't play Shepard and his party in these missions; instead, they create custom characters from a selection of races and classes. The multiplayer missions are an optional way to affect the outcome of the single-player campaign.
ME3 arrives on March 6th in North America. BF3 will debut here on October 25th.