No taillights, brake lights or turn signals

2 things to look at is the directional switch and or brake light switch on the brake pedel. my 1999 monte had the same thing and I replaced thedirectional switch and it was fixed

Have you tested the turn signal relay?

This is a common GM specific problem. Your turn signal switch contains an elaborate networking device that has to cancel brakelights while running a turn light. Often times the ground within the turn signal switch is lost causing your situation. After investigating such other possibilities as fuses and loose grounds it may become necessary to replace the turn signal switch. Thank you, Dana

Wrong replacement bulbs.

check blubs if the center light works should be working side brake lights

bad brake light switch!!

Turn signal switch. With you foot on the brake have someone stand behind the car. Wiggle the turn signal lever and see if the the brake lights flicker.

you give some good clues. because your third brake light works but not the other two, and because you sometimes get brake lights if you have turn signals on at the same time, i would say that some wires got crossed up with your other taillights. check out the power wires back in the taillight assembly and the wires that run back to the taillights. something is mixed up back there, now you just have to find it. if all else fails after you isolate which wires are the brake light wires get some new wire, same gauge and roughly same length, remove a bit of insulation but don't cut the wire in two, and put the new wire in as an overlay. if having the new piece of wire there makes the brake light work you found the break in the wire. you can cut out the bad piece and fasten the new piece in. you can solder it if you have the equipment to do that, or you can buy some squeeze in place connectors at the car parts store. either way, good idea to tape it up when you are done. hope that helps

Check the fuses under the hood, 99% of the time that's what the problem is.