I think 3 million would be possible. SM3DW, Nintendo Land, and NSMBU have sold 2, 3, and 4 million copies respectively thus far.
http://www.nintendo.co.jp/ir/en/sales/software/wiiu.html
The Wii U install base continues to grow (albeit slowly) so it should at the very least have the best debut on the system up to this point. Smash will grow the install base further. The Wii U should easily have it's best year (and holiday season) this year, though that's not saying much.
When all is said and done, I think it's very likely that MK 8 will reach sales of 5-6 million. That's still really bad by historic standards for the series but I think that's realistic given the Wii U's install base and the people who buy it. I mean, why would anyone buy the system but skip MK8 (or Smash).