Simple 10W/3V = 3.334 A. so the Mosfet Should be at least 10A Idmax and have a heat-sink capable of 40W dissipation because you should figure that there 'might' not be the heat transfer to ambient that you expect, and there are 3 of them... The Circuit will not work as shown however as you are missing a 10K pullup to the gate of the mosfetas shown. The NPN keeps a constant .63... Volt base bias by controlling the gate to source voltage of the mosfet and you can putt a 500 to 1K ohm resistor in the emitter of the NPN and adjust the current limit too.
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