DVDdoug:
You'll have to check the datasheet for the particular device. 12V is virtually never a problem and it's not hard to find one that can switch 3 Amps. (Give yourself some safety margin on the current rating.)But, do make sure it's a "logic level" MOSFET that can be switched/controlled by 5V. Many (most?) MOSFETs require a higher gate voltage to turn-on fully.
If you try to drive the motor directly from the Arduino (which puts-out 5V) the motor won't run and you might burn-out your Arduino. You'd actually get less than 5V and far-far less than 3A, but you'd get more than current than you can safely draw from an Arduino output pin (rated for 40mA maximum).
Actually no... You can connect it to a car battery (capable of hundreds of amps) and it will work fine. [u]Ohm's Law[/u] says Current = Voltage/Resistance. So although we don't know the "resistance" of the Arduino, and it will change depending on how many things are connected and if LEDs are on & off etc., we do know that current depends on resistance.Ohm's Law is a law of nature and it's always true, so if the resistance is too low and the "calculated" current is more than the power supply can deliver, something will "give". The voltage might drop, the power supply might burn-up, etc.
Alright, thank you that helped me to understand better the concept behind the MOSFET.
So what do you advise me to do about the MOSFET and this whole solution,
should I connect to the barrel jack connector my 12V-3A power supply?