The lowest voltage setting on the invader 3 is 3.6 volts.
Correct. But the battery cut off voltage (per cell) is around 3.2 volts. Can’t find the manual for the Invader 3, but the Invader 4 indicates Low Battery Voltage Error is voltage under 6.4 (series) volts. So that would be 3.2 volts per cell.