a three battery pack of 3.6V/750mAh
If this is from 1992, then this must be NiCad 3S (in-series) battery pack, 3.6 V nominal. As you said, the capacity is 750 mA-h. Every RadioShack used to sell replacements of this sort, 

 

There are still places where they can build a replacement pack with right size of cells. The charging voltage for a healthy pack should reach about 1.45 V per cell (~4.35 V total), and the charger should stop either when the cells start overheating, or by monitoring a subtle change in voltage increase, see BatteryUniversity aticle BU-407.

However, it is highly recommended to throw the whole thing away.