Great question — answering as the UphillTech team:
This is quite a the sticky wicket. So to be clear, there are four cables entering the light fixture, all with three wires contained in the cables. All four cables have green/yellow wires, three cables have brown and blue wires, and the last cable has a black and a grey wire. Most likely, the black and grey cable is your line voltage. I would take all of the connectors apart and take a reading on the black and grey to confirm. If you get no reading, try flicking on and off the light switch. If you have also switches at the bottom and top corridors to turn on and off the corridor lights, then it's quite likely that two of the other cables are the "traveler" cables for the two three way...
Full answer by Charlie Zxi: https://diy.stackexchange.com/a/268563 (CC BY-SA 4.0).
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