Ther are two main areas for this to happen with third gear on your Polo. Firstly, the gear change linkage: The linkage is connected to the gearbox selector shaft by a small 10mm bolt. This bolt can come loose after time, and allow the shafts to pass over each other instead of moving the selectors to fully engage gear. Third/fourth selection is in a straight line to the box and is more likely to slide along/over its detent. The rest of the linkage has rubber mountings, which can become worn and again prevent full engagement of the third gear. They are checked from under the vehicle with an assistant changing gears, engine off, for the loose bolt or worn linkage. The second area is internal to the box: Third/fourth hub and clutch select these gears and when selected are retained in place by a small metal detent(slipper). The detent can become worn and fail to cause the detent of the third speed, allowing it to 'jump out of gear'. A further cause is that all of the assemblies for gear changing are mounted on the gearbox mainshaft(third motion shaft) which has a shimmed end float. Third gear being on one end of the mainshaft makes it susceptable to any increase in end float(wear) and allows the third/fourth hub to drop third if wear in box/bearings allows excessive end float. A test of the box would be for noise. With the vehicle idling in neutral, press the clutch pedal to the floor. If any transmission noise goes away-goes quiet, you could have worn bearings. Now drive the vehicle on the road at about 30 MPH-50 KM/HR i third and allow the vehicle to coast(slow down by itself-foot off of the accelerator). If it jumps out of gear now then you have wear on the mainshaft or hub and clutch assembly.