I just bought a 1995 Chevy 3500 and I noticed that when I put the right turn signal the 3rd brake light also blinks and that when I press on the brakes all lights stop blinking I’m wondering if anyone can please help me out by the way everything works fine with the left turn signal

 

If the wiring hasn't been jacked with, replace the bulb socket.

 

Check to see if it's got a trailer light hookup. Sounds like it's grounding or shorting to wires in a harness or connector.