I had the same problem. I replaced all 3 switches since I had it apart, and the total cost was $35. Later I opened up the switch to see how bad they were. The top two black switches had considerable contact wear from sparking and arcing, leaving lots of soot and carbon on the switch walls. The bottom gray switch was like new when I took it apart, it probably didn't need replaced, but for $10 it was no big deal.

My situation was the switches were intermittent. The switches all tested good with a meter. During use, the contacts would occasionally stick apparently, causing the breaker to trip. FYI, if you have a 20Amp house breaker feeding the microwave, it's likely the microwave fuse inside the microwave will blow. That fuse is accessible without taking the microwave out by removing the vent cover above the door, exposing the fuse. Two screws on the top allows the removal, but it's tricky. However, replacing the fuse will likely result in another blown fuse without replacing the problem switches.