your circuit requirements are still a little foggy... what is on the switch common? power, or ground. If it's power, that's trickey with n channels, if it's ground, that's better... here's the problem... you have to get the gate x volts above the source. On the IRF510 the gate threshold voltage is 2-4 V. If you put the transisitor on the high side to source the 3A to your circuit, this means you have to get the gate at least 4 volts above the source. The IRF has a on Rds (resistance drain to source) of .54 ohm. At 3A, that means the transistor will drop about 1.62V across it. This means the gate has to go to 2.32V above the drain. If you've got V+ connected to the drain pin, then this is your highest voltage. You would need what they call a high side driver to turn the mosfet on in this manner. You also have a bigger problem. At 1.62V x 3A, you're gonna dissipate about 5W! Or 2.5W at a 50% duty cycle. I hope you also have a heatsink.
If you put the transistor so that it completes the circuit's path to ground, this is the normal method an n channel was designed for. You'll still have the heat problem, though.