Welcome to FlyerTalk, striker1983! Yes, your same-day departure boarding pass will allow you entry into any terminal. A BP checker might tell you that you're entering the wrong terminal, but just explain that you are visiting a lounge.

IMPORTANT: the Amex Centurion Lounge is in Terminal 3F, not International-G. It's very close to the airside hallway connecting T3 to I-G, across from gate 74. You could enter security at I-G and that's a good idea if the line is short. If you do that, make an immediate right turn after security, following the signs to Terminal 3. If the line is long, you might want to make a right turn before security and try your luck at one of the Terminal 3 security points.

Safe travels and enjoy the Centurion Lounge. It's very nice, but often quite crowded. I suggest leaving the lounge no later than 40 minutes before your departure to assure that you get to your International-A gate on time.