Controlling a 12V coil using a 3.3V MCU
With MOSFETs, one of the important design parameters is how much gate voltage is needed to turn the device on.  

This particular device is not being driven very hard with only 3.3V on the gate.  It's probably fine at 3A, though.  R1 is okay, but R2 is way too small.  It's not needed while the gate is actively driven; it's there for ensuring the device is off when the driving circuit is in an un-powered state. You want the gate to be close to 3.3V, so around 10k ohms would be better.  

And like someone noted above, D1 is too small.