The third chamber starts off with a mini open puzzle leading to the actual test chamber itself. So begin by dropping down off of the walkway, running across the fallen metal girders until you reach the tall concrete pillars.
Look up high to your right and you should spot a metal walkway wrapped around the top of a pillar. Zoom in and fire your first portal up here at the wall. Now place the second one on the nearest white wall (such as the one on the middle pillar to your left).
Now position an exit portal on the slanted wall panel by the force field and the run around to the other side. Place an entrance portal back under the gel dispenser and watch as a large blob gets flung across to the large platform leading to the exit. Now drop off the platform and quickly place your entrance portal below you so the speed carries you across to the blue gel and up and over to the exit.