Trapped and gassed!

“Ouch!” I moaned as I slowly regained consciousness. Someone had knocked me out cold from behind. I now found myself alone, trapped in a small square room. The floor was hard and cold, and the room was empty save some strange Mondrian-like paintings. My assailant clearly had a bad taste in art.

Unfortunately, this wasn’t his greatest flaw. As I was about to discover, he was also quite sadistic.

“Welcome back, lieutenant,” came a raspy voice from some hidden audio system. “You have been quite a nuisance.”

(That was an understatement. I had destroyed his laboratory of evil inventions.)

“But now,” he continued, “the time has come for us to part ways. Permanently.”

I scanned the room, looking for a way out. On one wall, there was a sturdy, metal door with a security keypad. One of the adjacent walls contained a reflective panel, presumably a two-way mirror so my tormentor could watch me. My thoughts were interrupted by a hissing noise.

“This room is starting to fill with a poisonous gas. You have approximately three minutes till your demise,” he chuckled evilly.

I began ripping the paintings off the walls, looking for anything that might assist me to escape. Nothing! The wooden frames weren’t very strong, so there was no use trying to smash the glass. Gas continued to vent into the room. I could feel my lungs burning. Suddenly the light went out. The darkness was temporary, as it was soon replaced by a pulsating light, which when combined with the gas was extremely disorientating.

I was just about to admit defeat and succumb to the gas when I saw it. My tormentor was evil and sadistic, but his disposition to give me a sporting chance may have been his one redeeming quality.
I held my breath and forced myself to the door. I punched the hash key followed by the three digits. Success! The door swung open and I was greeted by fresh air. Time to run!

What keys did I enter to escape? How did I work it out?

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Hints:

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See @Stiv’s answer below for solution. The following is some extra explanation: