Specification Clarification for FETs
From your other question you need to be able to switch 3A and you need to switch 3A so that when the MOSFET is on it isn't going to get too warm so maybe say &quot;it shouldn't dissipate more than 100 mW at 3A. This means its on resistance (\$R_{DS(on)}\$) is a maximum of:
\$\dfrac{power}{current^2}\$ = 11 milli ohms.
Next you need to be able to turn that fet on down to 11 miili ohms with the gate voltage you have available - maybe this is 5 volts or maybe it's 10 volts - you have to work that out then look at a the graphs on the data sheet for several FETs and decide if the gate voltage you can supply is ok for switching it. Here's an example of the type of graph to look for: -

I'm not saying this FET will do the job, I'm just pointing out the graph you need to look for. On the graph above, with a gate voltage of 3.3 volts and a drain current of 3 amps, the drain source voltage will be about 0.45 volts meaning, the on-resistance is 0.45/3 = 150 milli ohms.
Yes it's a little tricky for a novice but you will be doing your self a favour finding the FET yourself and gaining that knowledge you can use countless times over (well maybe a few!).
Also make sure your voltage rating is well above what the circuit might generate and don't forget a reverse biased diode across the coil rated at least at 3 amps.