On a 3 wire sensor one wire is ground, one is o2 signal to the ecm and one is 12v positive. The ground and the 12v pos are for the heater inside the sensor. The only reason for that is to heat the sensor sooner allowing the ecm to go into closed loop sooner for emissions reasons. The original sensors only had one wire, so if that wire is part of the heater circuit you could actually use it as it is if you wanted. I'm sure you could look up the wiring schematic online.