As others have said, the lounge situation at SFO is rather dire... I'm sorry that you have to spend three hours in any of the lounges, IMHO this airport is crying out for a single Oneworld lounge like LAX.
Anyway, that said: if you're arriving into SFO three hours prior to QF74 then you should have a shot at getting into the CX lounge, which is easily the best of the bunch. CX has tight limits on how many QF passengers it will allow it, due to concerns of over-crowding in the lead-up to its SFO-HKG flights, but this should be your first port of call.
If CX denies you access, well, not much to be done but head for the Air France/KLM lounge which Qantas uses, or just browse the terminal and see if there's not some public cafe or such that appeals more.