There are BC337 NPN transistor between MCU & 3W LED
If the Arduino is getting hot, something is wired wrong.
Is there a resistor between the transistor base and the Arduino? The base-emitter junction is effectively a diode and if it's not current limited, you'll draw excess current from the ATmega chip, possibly damaging it.
Of course the LED is also a diode and must be current-limited. Normally, a special constant-current switching supply is used for LEDs of 1W or more. With "regular" LEDs you can use a resistor, but that's not efficient with high-power LEDs and you'd need at least a 5W resistor because the resistor will get HOT!