A million sales in four months in Europe. 10 million sales worldwide, let's attribute 8 million of those to Europe. 40 million PS2's in Europe, that's 20% in seven years. How can one look at that relatively huge success for a peripheral and consider 10% penetration for Kinect in one Christmas is a likely outcome?This peripheral discussion keeps coming up, but there are plenty of peripherals being overlooked. As mentioned The balance board but there is also the original DualShock. Even the EyeToy did much better than people think, if you just look at Europe.
As for Dual Shock, it was successful enough to be come the default controller, which is going to help install base. Did it sell to 10% of PS1 users over it's first Christmas? Can't find any numbers. It's also something of a fringe case as it was an upgrade to the existing controller and not a standalone device. Every PS gamer would have had an interest in it, unlike other peripherals that aren't intrinsically going to appeal to the existing user base who bought the console for a different input format.
All in all, that 3 million expectation doesn't seem to have much statistical basis to me. There may be the market research to support that estimate, but not the historical precedent.