Q: My third break light comes on when I apply the brake pedal, but my two main brake lights dont

Hello. Thank you for writing in. The working brake light is known as the center high mount stop lamp. It is wired differently from the normal brake lights in order to prevent them from all going out at once. The normal tail lights are wired through something called the combination or multi-function switch. You should test you blinkers as they are wired through the same switch. It is likely this switch that has failed, and it will need to be replaced. If this is not the cause, further testing with a voltmeter must be done to find where the voltage supply that is supposed to power the bulbs is being cut off.