Calculating area of polygon in WGS84 Shapefile using QGIS

To change the CRS of a layer you must not edit it in its properties but "save as..." and choose the CRS in that dialog. You will get a new file.

In QGIS if you calculate areas it always uses the units of the layer on which you operate. So in your case it tried to calculate some angular area garbage. ;) Save your file as GK3 as well and then use that, you will get square meters.