Can I use USB to power a flashlight that runs on 3*AAA or 1*18650 battery?
From your picture it looks like the LED driver has some active electronics, most likely a primitive PWM regulator. Which means that it is not critically sensitive to input voltage if it is above the LED voltage threshold. 

The specs also say that it will run for 14 hours, which means that it has a 200 mW LED (assuming 3xAAA = 3 W*hr), and it will take something 1000/14 = 70 (x inefficiency) = maybe 100 mA from the source. So it looks like it is pretty safe to run this flashlight from any USB port.