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Factory wiring diagrams only show two wires at both of the cigarette lighter ports. Should be an orange and a black. Red, yellow, and black sound like aftermarket wiring colors, though even with an aftermarket I can't imagine why you'd need three unless it had some sort of built-in illumination.
From: Jacksonville, FL (originally from Toronto Canada)
I assume you're looking at the universal lighter splice harness you posted about in another thread? That harness uses a connector which is standard across most GM products so HappySalesman is probably right that the extra wire is for those models which had an illuminated ring around the lighter socket. In your case, the two wires you need are the ones connected to pin A (power) and pin C (ground).
A = Battery positive (orange)
B = Unused - probably illumination on some models
C = Ground (black)
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THREE wires go to cigarette lighter? 
