The third Brake light on my 1994 chevy truck works properly but the back two will not work. The signals work fine also the hazards work fine. Checked the brake pedal switch, getting power on the yellow wire there but nothing at the plug at the bumper while pressing the pedal. But the signal light is sending power down the yellow wire. Don't know where to go from here. Thanks for any help that's out there.

Replace the bulbs. They have multiple filaments so in all likelihood the brake light filaments are burned out.

If the bulbs are good, try your 4 way flasher, walk around and see what lights, If the rear flashes, bulbs and wiring are good to signal head. On many, third brakelight comes directly from brake switch, since the left and right brake light come from Signal head, I would check at signal head.
Can't find a diagram for your vehicle, so this may not apply. It can also be done with 2 diodes.