Break out the multimeter. Start at the bulbs. Verify the ground is good and then and work your way back towards the fuse box looking for an open as long as the wiring is accessible (Have a friend / brick depress the pedal). Assuming you haven't found the trouble spot, move to the fuse box and go the other way. If you're lucky the problem will be where you can get at it. If not, it will be after the wiring you're interested in has disappeared into a thick bundle and / or gone under the carpet. Before you tear out the interior, figure out how the center and right lights are wired and how the wires are routed. You're probably looking for one fault that took out both circuits. Hopefully that narrows down what you have to remove. If you determine the fault is likely in the middle of a a thick bundle of wires, consider just running new wires instead of tearing the harness apart. Good luck.
It might be worth checking the brake switch with a multimeter. You know it's completing once circuit, but what about the other two? I guess it depends on how it's wired. Check the wiring diagram.
Don't assume the bulb sockets are good. I've chased intermittent problems that survived many scrubbings with cleaners and applications of dielectric grease that were resolved by changing the socket. If it's discolored due to heating, strongly consider changing it, especially if you can get a replacement for a decent price.
