Considering there's just under 10 million Wii U owners and that this is a new IP, I can't see it selling 3 million units in its lifetime.
I think anything from 750K - 1 million lifetime units sold will be a success. I think it'll fall between 350K - 750K lifetime units sold, which I think they'll be 'content' with.
But, I'm really hoping Splatoon kicks off and gets more owners behind the Wii U. It's a hella' fun title, and its one of those silky smooth experiences only Nintendo can offer.