A lot of the more sophisticated Ni-cad chargers could charge ni-cads at higher current levels thus performing 'quick charge' Vs the older C/20 trickle charge method. However for this to be safe the charger really needed to be able to monitor the temperature of the Ni-Cad pack being charged, and that is most likely what that third wire/connection is a raw temp sensors that the charger reads to determine if pack is getting too hot to continue fast charging.
At least that is my guess of what you have.
Lefty