I travel via LAX lots of times. If you check in your luggage in HK all the way through to Seattle, your luggage tag will have HKG-LAX-(Seattle airport code). You have to pick up ur luggage and clear customs in the international terminal. After you exit customs, there will be conveyor belts for you to put ur luggage, and they will be transferred for the flights to Seattle. Then, you have to exit and take the bus (driven) to the right terminal for Alaskan air. The bus is circular.
From my past experience, 3 hours should be enough. But of course, it depends on immigration.