To get flights from any of those three locations you'll need to use the ITA Airfare Matrix Search.
Once there click "Advanced routing codes." Then enter the city you're departing from (SEA = Seattle, PHX = Phoenix, DFW = Dallas). In the slot that says "Advanced routing codes" enter "dl" if you're leaving from Seattle, "aa+ /-overnight" if you're leaving from Phoenix, or "dtw,sea /-overnight" if you're flying out of Dallas.
The destination code for Beijing is "PEK" and Shanghai is "PVG." The next slot is "Return advanced routing code," which needs the same code you placed in the first "advanced routing code" slot.