1999 Chevy Venture no brake lights Third brake light works you have changed bulbs and brake switch.?

Part of Brake Light System Nonfunctional
The fact that the "third" brake light does work tells us that the much of the brake light system is good and thus working. This tells us the fuse is good, the brake light switch is working, and that the wiring to the 3rd light is good.

The fact that the 3rd light works and the brake lights at the rear don't work tells us that the specific defect causing your problem is located somewhere BETWEEN where the wires for 3rd light split off from the main harness, and split/fork for the rear brake light assemblies.

Although I'm not familiar with your vehicle, that year most, if not all, vehicle brake light systems had the main harness go to one of the rear tail light/brake light assemblies, and then "split" off with the harness continuing to the other tail-brake light.

That suggests that the OPEN CIRCUIT condition [your defect] is located in that section of the circuit between where the 3rd light wires split off from the harness, and where the harness splits for the two rear tail-brake light assemblies.

We're only talking about roughly 5 to 8 feet of wire/harness here, and that's where to look for something like a disconnected or "loose" connector plug, or a cut wire.