Usage and charging of 2S Li-ion batteries
The third wire (as in, the one which is not the + or - battery terminal wires) will be a built-in PTC (positive thermal coefficient) thermal resistor, or "thermistor". These are used in closed loop temperature monitoring by charge controllers. This battery has no balance charge leads. Most chargers will slowly and carefully inject current while monitoring temperature until the current drops off to a point (or after a timer) where it switches to voltage mode and leaks the last amount of charge into the cells. Without a balance lead to help the controller do the control or per-cell injection, it just has to assume the cells balanced themselves over time. 

So the main point is, that extra wire is a thermistor which a charge controller can read to do temperature monitoring of the battery pack. This is critical because Li-Ion batteries are susceptible to overheating and exploding while charging so these thermal feedback mechanisms are important for safety, though not actually required of course.