I really believe the jumping out of 3rd gear is caused by a combination of things moreso than by one particular problem.
In rebuilding my 3 spd, I was able to find some new gears to install, but also had to try alot of "good" used parts before it was right. Good detents and forks help, but if the inner shaft doesn't mate up perfectly inside the drum gear, it still seemed to jump out. Lots of slop in the bearings and gears ain't good either. The whole thing needs to be as tight as possible.
