i dont have one so i cant say much but if it draws 3 amps it will suffer from heat and heat sag as there is very little contact with the head going from the pictures compared to the Trustfire TR-3T6 even the description for the UniqueFire 3900 says it can not be ran on high to long ? we know the TR-3T6 has a good driver (buck + boost with constant current) with a soild screw in and it will handle long runs, but the UniqueFire 3900 looks good with its stainless steel bezel.
the heads on the UniqueFire 3900 and the Trustfire TR-3T6 both are 55mm wide and the TR-3T6 screw in is just over 50mm wide (top) and just under 45mm long (deep) the reflectors on the TR-3T6 are 17.30 mm deep and 22.50 mm wide aprox
hope that helps