Originally Posted by Stripe

To be clear on the directions, there is a long bridge connecting the C and G concourses. In the middle of that bridge there is an exit down to the train station. Buy your ticket at one of the machines. Since the tickets are valid for 2.5 hours after purchase you can use the same ticket on your return trip. When you return to the airport you go back up and through security in the same place you exited (in the middle of the C-G bridge).

If you are a SkyClub member, it is convenient to store your carryon luggage in a locker in the C club, especially if your onward flight is departing from there.

A 3 hour layover should leave you with about an hour at the mall. To be safe I would be on the train heading back to the airport about an hour before departure.