Join Date: Jul 2007
Location: San Francisco/Sydney
Programs: UA 1K/MM, Hilton Diamond, Marriott Something, IHG Gold, Hertz PC, Avis PC
Posts: 8,156
Unless you are mobility-challenged,do NOT take the air-train for this connection - it'll be faster to walk it.
From Terminal 3, follow the signs to International G terminal. This will take you air-side along the connector between Terminal 3 and International G.
As soon as you get to Intl G (you'll come up the escalators and then see the security area), turn left and take the exit. Then keep walking straight and you'll come to the Intl A security area (after walking past the check-in counters).
Your biggest risk here is that UA likely won't be able to give you a boarding pass for the QF flight, and QF check-in closes 60 minutes before departure. You may be able to check-in online, but if you're not there's a real chance you won't get to the check-in counter before the 1 hour mark.