I WILL buy it for the PC,from any legal source...Except Origin. I have a VERY bad track record with EA Online. Example: My EA Online account was logged out in the middle of me playing through Operation: FIREWALKER with Zaeed in my party,using the Arc Projector,wearing the Cerberus Combat Armour.
Now,all of that is Cerberus Network DLC. Which is tied to my EA Online account. So,I'm bounced in the middle of combat because the game thinks I don't own the DLC. When I try to log back in,I discover that it,SOMEHOW from the Logon screen,created an entirely new EA Online account for my email. So,my Cerberus Network DLC is now lost forever.
Because of that incident,I now adamantly refuse to use EA Online,or anything related to it. I will,however,play games that have an optional EA Online component. I'll just stay away from EA Online.
So,TL;DR...
I WILL get ME3 for the PC,if not from Steam,then from retail. EA Origin/Online lost my faith.