Unfortunately this used to be a common problem with the Super 5 jumping out of 3rd gear, and is caused by wear on the gear selector drum, to repair the engine needs to be completely stripped and maybe a new selector drum fitted,
Burwins in London can repair the drum but parts can normally be found on ebay quite cheaply, or you can ask members of the MZ riders club as most members have a stash of parts they may want to sell,
the drum, gears, selector forks, are common to all 5 speed 250-300 MZs, TS, and ETZ, if you do this yourself make sure you replace the gear lever return spring, this is also a weak point and is also located inside the gear box.
BUT FIRST check your CLUTCH is adjusted as per the works manual.
http://www.mzridersclub.com/