Charging 3 NiCd AAs in series
It seems good to me,

A C/10 charge for NiCd batteries is recommended at a rate equal to one tenth the mAh capacity (100mA in your case)  for 14 hours. so charge from empty for 14 hours.

The battery requires a DC supply to charge, you'll need to convert the AC somehow. I guess you can re-use whatever setup the old battery used.