Generally a worn syncro will cause it to pop out of 3rd gear but you have to look at some of the other parts also. The clutch gear could be worn where it makes contact with the syncro, the tip can also be worn where it makes contact with the pilot bearing and the pilot bearing can be worn. In addition, you could have a worn 2-3 shift fork. While you are at it check your U-joints, the input yoke and the bushing in the rear of the trans. All of these can be contributors to the problem.
Note: The parts for your trans must come from 55-67 car or truck, with the same configuration, nothing earlier or later will interchange.
Fred
