I acquired an ETZ engine recently (same gearbox) which was described as having a very similar fault to yours. Difficulty in selecting 3rd gear and frequently jumping out even if it did select.
When stripped I found that the circlip that holds 3rd gear in place had come out of its groove. It had moved about 6mm towards 1st gear. My diagnosis was that this prevented full selection of 3rd gear most of the time hence jumping out. Occasionally it would select sufficiently for the undercut to pull the gear fully engaged but not enough to pull the circlip back into its groove. Next time 2nd gear was selected it pushed the circlip back again so repeating the cycle.
I have yet to establish why the circlip came out of its groove and it does not appear to be worn. Nor is there any serious wear on the bits that normally cause 3rd gear jumping problems so running with it like that does not seem to have caused any collateral damage. However, I am not confident enough of the diagnosis to rebuild the engine as yet so cannot prove my diagnosis.
Regradless, sooner or later you are going to have to strip the engine to investigate.