It would be safe to say that you can toy around with the wires, since this is a low amp and low volt device, you will not suffer from contact with the output wires.
I would reccomend testing with a 12volt fan, but with the adaptor set at 3-6volts. Trial and error. When the fan moves, you have found the correct setting. (I am about 50% sure that one wire is common, and the other two are +.) The one in the center or with a white stripe may be the common (-)... The power adaptor allows a polarity switching which is what I think the third wire is for.