Kind of an edit after my post:
I found the actual ratios of Gears of War vs. Halo and Halo 2.
Although Halo 2 actually increased in sales by quite a few units (about 1.5m globally), the ratio of european/Japanese sales decreased quite a bit. On the other hand, Gears of War's ratio is FAR better. Some of this could be attributed to the fact Europeans prefer TPS over FPS more.
However, this leads me to extrapolate the following scenarios for your enjoyment:
Scenario #1:
Halo 3 sees the exact increase in sales that Halo 2 saw over Halo 1 globally
9,350,000 Units sold Worldwide
Scenario #2:
Halo 3 sees the exact increase in sales that Halo 2 saw over Halo 1 in the USA/NA (7.3m sales, a 1.2m increase over h2), but see's a 2:1 ratio of NA sales to European sales, mimicing Gears of War
10,965,000 Units sold Worldwide
Scenario #3:
Halo 3 sees similar US/NA results to Halo 2, but sees a 2:1 ratio of sales in Europe
9,165,000 Units sold Worldwide
Scenario #4:
Halo 3 sees similar US/NA results to Halo 2, but sees a Halo 1-type ratio of sales in Europe (about 10% higher)
7,997,990 Units sold Worldwide
I'm not saying ANY of these will happen, but I do think one of these scenarios is likely to happen. IMO, I'd say scenario 2 is the most likely, but who knows.