Welcome to AO where threads must wander of topic sooner rather than later
I hope you car is still under warranty
It could be five different things really;
1) worn Syncronizer and sleeve assembly
2) worn selector fork
3) gearselector locking mechanism faulty
4) too much play in input shaft bearings
5) worn needle bearing on third gear
As Hoygaard said it could be many things.
I had it on my Sud once, the selector fork had slipped on the selector shaft so when the selector shaft was in the detent for 3rd gear the fork hadn't pushed the gear fully home.
It would hold 3rd gear fine until you lifted off the throttle slightly, at which point it would jump out.