Return to the hallway at the bottom of the slope. Step on the one floor tile without the gold pattern on it. Lara is standing on it in the screenshot above.
This pressure pad opens a concealed door at the very top of the ramp where you fought the three mutants. It's on the right, just before the hallway narrows. The door stays open for about 25 seconds, giving you ample time to reach it. Inside is secret #3.
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