Well we know for sure Halo: Reach passed 10 million including digital sales, it was bundled with Fable 3 and that got a 1m+ boost due to the bundle, Halo: Reach must be around 11 million by now including digital.
Halo 4 is still selling well, will probably reach 10m at retail eventually and well over that including digital.
There may be a slight downward trend but it's nothing huge, it's pretty insignificant to be honest. I expect Halo 5 to sell in line with Reach, I doubt 3 will ever be matched simply because it was on sale for so long, released before the rise of Call of Duty and the Xbox 360 did so well.







