2002 Buick Century Stop light failure

I own a 2002 Buick Century with 72,000 miles and a v-6 engine. I have recently discovered that the right rear stoplight and the 3rd stoplight above the trunk do not function when I depress the brake pedal. However the left side stoplight does function when I depress the brake pedal. The vehicle is equipped with daytime running lights. When I manual turn the headlights on the taillights light up. But even when the headlights are on the and I depress the brake pedal the right side stoplight and the 3rd stoplight do not light up. I have checked the fuse and circuit breaker for the stoplights and it is intact. I have also examined the right stoplight and the bulb is working. I did not however examine the 3rd stoplight. What could the problem be?