[markeb01]

I've never had a 3 speed pop out of 3rd so I looked up the troubleshooting section of my 1966 Motors Manual. They show 2 types of transmissions. I'll list them both since I don't know which yours is.

A. GM transmission with 2nd speed gear located at rear of mainshaft.
1. Transmission mounting bolts loose.
2. Control rods interfere with engine mounts or clutch release lever.
3. Control linkage does not work freely.
4. Gear does not fully engage.
5. Damaged mainshaft pilot bearing.
6. Clutch gear bearing retainer broken or loose.
7. Dirt between transmission case and clutch housing.
8. Misalignment of transmission.

B. All transmissions with low-reverse located at rear of mainshaft:
1. Improper shift procedure.
2. Linkage parts worn, bent, broken or out of adjustment.
3. Excessive end play caused by wear in shift forks, sliding gear fork grooves, thrust washers, mainshaft and countershaft bearings, or clutch pilot bushing.
4. Misalignment or excessive clearance between sliding gear and mainshaft.
5. Damaged synchronizer.

My experience with a manual trans popping out of gear has always been a bad synchro, but it's normally on 2nd gear in a 3 speed, or 2nd or 3rd in a 4 speed. Hopefully something hear will help diagnose the problem.