Even though the problems with 3-wire are rare, I'd rather be safe than sorry-- especially with the kind of luck I have. Not too long ago, my microwave's body & door became electrified with a low current & gave me a little zap. I had to unplug it & move it to a different outlet. I think I'm going to replace all of my receptacles with new ones bc most of them are well over 20 or 30 years old and have been through numerous power surges.
I'm concerned about your microwave problem. What more can you tell me about that receptacle outlet? Have you tried any three wire tester on the outlets in your home? What did it show on the microwave's previous receptacle outlet.
Do you have access to a Wigginton type solenoid voltage tester like the one shown here?
Solenoid voltage testers, unlike neon testers or multimeters, place a small load of about 7 milliamperes on the circuit under test. They are thus immune from false voltage readings caused by inductive pickup of a false voltage from adjacent conductors and will only show a voltage when there is actual energy on the conductor under test. By placing one lead to the round pin terminal and the other to the shorter straight terminal you will find if the ground connection is good enough to actually carry current. Then by testing the resistance between the longer straight slot and the round pin opening with a multimeter set to OHMS you can get an idea of whether the connection between the 2 points is several feet away in the panel were it belongs or bootlegged right in the box to give a false OK reading on a simple plug in tester. A measurable resistance suggest a proper Equipment Grounding Conductor path back to the Main Bonding Jumper in the Service Equipment enclosure and out on the Grounded Current Carrying Conductor to the long blade of the receptacle. An extremely low resistance suggest a bootlegged jumper in the box under test.
Without some experience using a solenoid voltage tester even it cannot tell you if someone deliberately attached the Grounded Current Carrying Conductor to the Equipment Grounding Conductor terminal of your receptacles in order to deceive the simple 3 light tester. The only way to detect such shenanigans is to open up the cover plate and pull the receptacle out of the box and check for the presence of a jumper, as shown in the picture below, between the green colored terminal and a silver terminal or a splice of white wires OR to apply a multimeter to the outlet as outlined above.
A circuit analyzer, shown below,
would be easier to use but they are expensive costing more than 300 dollars when new. The photograph shows a bootleg ground jumper with a normal reading on the three light tester and the bootleg ground detected by the circuit analyzer. The circuit analyzer does the comparative measurement which I outlined above but it does it automatically. The three light tester detects a ground connection but the circuit analyzer tests the quality of that connection.
You should not have gotten any shock off of the skin of the microwave unless it is internally faulted even on an ungrounded circuit. You can eliminate that possibility completely by measuring the resistance between each of the straight blades of it's plug and the EGC round pin. Any OHMS reading other than infinity, meaning no measurable connection, indicates an internal fault in the microwave.
With a bootleg ground the microwave could be in perfect operating condition and by connecting the shell of the microwave to the Grounded Current Carrying Conductor could raise the voltage on the surface; which is called the touch potential; high enough to cause a shock sensation. The volts used in moving the current through the the Grounded Current Carrying Conductor back to the panel; commonly called the voltage drop; can be enough to cause a shock. The poorer the condition of the Grounded Current Carrying Conductor the greater the voltage drop will be.