Yeah the 3 OS thing needs to be cleared up.
I think MS decided to use that line simply to cut down on the technical jargon they would have to include in their pr statements.
If you look into it further, you find out that the reality is not straightforward. You have a Hyper V OS environment that runs two virtual machines or OSes. One is the Xbox OS and the other is Win 8. The Hyper V part isn't an os in the traditional sense, its more of a virtual os manager in the background.
So the reality is that there will be 2 OSes running on the box, Xbox OS focused solely on gaming, and Win 8 to handle everything else.
I agree with others that the 3gb rule is in place for multitasking reasons, not due to any OS needing it performance wise. I'm all for upping the total ram, it just makes it more future proof.