Close enough for voltages?
Li-ion batteries range from about 4.3V - 3.3V as they discharge. 3xNiMH batteries would range from about 4.2V - 3V as they discharge. Too low of a voltage is not a problem, the camera already will have circuits to not turn on if the battery drops too low. All it means is you might not get the full use out of the NiMH batteries as compared to the Li-Ion, but there wont be any harm to anything.