3 NiMH batteries will provide 3.6V nominal. Fresh off the charger, you'll probably see something like 4.1V unloaded, which seems to fit with your observed 3.9V. I'm not sure what the voltage drop is at 3.6V and a 1-2 ohm coil, so I can't help you there.
I can vape 2.4 ohm coils without issue on...