Chamber 3

Set your first portal to the left of the beam receiver located on the upper-left part of the ceiling.

Place the second portal on the nearest wall so you drop down onto the upper platform where the Displacement Cube is resting.

Place a new portal directly underneath the beam receiver.

Look at where the laser beam on the left-hand wall is pointing. Place your second portal here. This will allow the beam to reach the receiver and open the first of the two locks.

Pick up the Displacement Cube and jump across onto the platform in the center of the room.

Place the Displacement Cube so the laser is deflected right onto the last receiver, opening the final lock and allowing you to proceed.