When I had my 3-speed rebuilt ('52), I mentioned to the rebuilder that dropping out of third was a common complaint with these transmissions. He was aware of the issue and had a fix. I can't fully remember what the fix was (but I think I mentioned it in an old post...try searching for it). Basically, a notch or grove was machined all around the cluster gear. This notch retained the gears in mesh until the clutch was released and load removed. I was worried it would make the transmission feel notchy, but he assured me it would be imperceptible.
I seem to remember he said the problem was due to the minimal engagement of the gears and wear of the cluster gear due to weak synchros, etc.
