If you want to play Mass Effect 3 on PC you will have to download, install and sign-up to EA’s Origin service. The game will not be available via Steam at launch:
“During initial release Mass Effect 3 will be available on Origin and a number of other third party digital retailers, but not on Steam at this time,” BioWare’s Chris Priestly said.
“Steam has adopted a set of restrictive terms of service which limit how developers interact with customers to deliver patches and other downloadable content.
“We are intent on providing Mass Effect to players with the best possible experience no matter where they purchase or play their game, and are happy to partner with any download service that does not restrict our ability to connect directly with our consumers,” concluded Priestly.
EA has also said that Mass Effect 3 will need a one time activation, as opposed to an always-in iternet connection.