I'm assuming the game is about to get a marketing push/info blowout. There is supposed to be a multiplayer beta test(limited though). If everything looks good, and reviews well it should sell good. 3 million shipped first week is doable.
Halo MCC will sell 5+ million copies(including digital)
halo 5 will sell 10 million copies(including digital)
x1 will pass ps4 in USA, and UK.