How the brake light switch work inside?

There's a switch on the brake pedal that when you step on it, it completes the circuit for lighting up the brake light.

fuse, bulbs

If you have the same bulb for brake and signal, brake light power needs to be interrupted by the signal lights, The brake circuit has to go through the signal light switch so the problem could be...

on the bulk head just above / behind the brake pedal. Should be a white plastic switch with two wires coming out the bottom

There could be many reasons. The most common reason would probably be that the bulb in the third light has blown. Try replacing it with bulb from one of the other lights or a new bulb and see if it...