U3 HAS been massively bundled starting a couple months ago; however, before the bundling started, it was at around 4.5million, outpacing U2 by about 800k. It had also just started to pick up in sales to 20+K per week directly prior to the bundling.
U2 had a 500k or so 2nd Christmas, so u can only assume U3 would've done the same or better without its bundling. So...without that bundling, U3 would likely be at around 5.05-5.20 million in sales. So it definitely would be much further from Gears 3's sales if it hadn't been bundled, but I still think these games belong in the same class, saleswise. Plus, seeing as U2 is a 6mill+ seller, and U3 was tracking nearly 1mill ahead of U2 pre-bundling, there's a decent chance U3 would've caught Gears 3 eventually even without its bundling.
The only unfortunate thing is that Gears 3 still made more money due to selling more copies of Gears 3 while it was still full price.