Brake lights not working, checked fuses all good

So you now have the regular brake lights but no third brake light? This sounds like the brake light switch or a fuse is out these guides can help us fix it. The fuse location for your can are in the diagrams below.

https://www.2carpros.com/articles/brake-lights-not-working

and

https://www.2carpros.com/articles/how-to-check-a-car-fuse

and

https://www.2carpros.com/articles/how-to-check-wiring

Check out the diagrams (Below). Let us know what happens and please upload pictures or videos of the problem. Here are the brake light wiring diagrams so you can see how the system works.

third brake light works, but the regular brake lights dont

Ahh. Ok. I switched them. Check the ground for the lights. Check the power lead for the lights. I can't visualize the rear-end of a Trailblazer right now, but I believe the brake lights are also the turn signals? Do those work?

all lights work except brake lights and now the third brake light isn't working either

Ok, so turn signals are not also brake lights? Have you checked all connections and grounds? Checked for power at the light pigtails/sockets? What about taillights (the ones that go on with headlights)?

thanks for the guides it turns out the ground was loose at the back of the car, all fixed

Glad you could get it fixed, that kind of problem can be tough. Please use 2CarPros anytime we are here to help.