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Double_J
Airhead
That's pretty common on lighting circuits, usually the 'extra' wire is used for a strapper in hallway 2 way switching scenarios. It's possible that they only had that wire available and one of your conductors is unused, in this case it would likely be the brown and black carrying line and neutral. You need to test them with an appropriate voltage tester.
It does sound like you are a little bit out of your depth though, don't assume that the colours of conductors mean anything in a lighting scenario. You need to understand and double check everything and work with the power off.