DIAGNOSIS: F3 is a door switch error. The microwave's control board is detecting a problem with the circuit of the three door safety switches.
FIX: Unplug the microwave. The primary suspect is a failed door switch. You'll need to remove the outer door panel to access the switch assembly. Test each of the three micro-switches for continuity with a multimeter—one will likely show as stuck open or closed. Replace the faulty switch. This is a very common repair for this series.
IF THAT DIDN'T WORK: The wiring harness connecting the switches to the main control board can have a pinched or broken wire. Inspect the harness for damage, especially where it bends near the door hinge.
HEADS UP: Never bypass a door switch. All three are critical safety devices that prevent the microwave from operating with the door open. Always replace with the correct part.