The title is a bit misleading since none of the three big boys mentioned are making just motherboards, they all have a full line-up of notebooks/netbooks, even mobile phones and some other peripherals.
For Q4, I believe the winner is going to be the one who manages a better bang for the buck in the netbook & notebook departments. Also, with Intel's new Core i5 out, the holiday season might end up with a lot of buyers of P55 motherboards.
What I believe will be sought after a lot in the following months are CULV based notebooks and cheap ION netbooks.

yep the netbooks must play a large part in these profits.

I once was an Asus fan (not anymore) and I have really noticed their presence is growing stronger in every online shop catalogue. From motherboards and net/notebooks to cpu coolers, there exists a large array of product categories created by this company (or by partners under their brand). One of the problems with this 'shotgun' approach is that product supports becomes more and more a complex and dispendious task.

Sometimes this fact becomes evident: I own a D2/PM sound card and even though it is a remarkable piece of engineering, it never had a decent WHQL signed driver. The majority of the released drivers have the designation 'beta'. Windows 7 is not correctly supported, while Realtek's onboard cards have already been supporting it flawlessly for months.