That's a link to a LED strip.
A LED strip has LEDs, and also resistors to limit LED current.
The mosfet is just a switch, like a relay contact.
Turning the strip on, or off, or on/off very fast (PWM).
Switching a LED on/off faster than the eye can see is percieved as a dimmer light.
So why the question about a 3volt/3Amp power supply. To drive single power LEDs?
You can't just power a LED with a 3volt (constant voltage) supply.
Something else is needed to control/limit the current.
Leo..