Strong enhancements of the upper thermospheric total mass density were observed by the CHAMP satellite at approximately 400 km altitude during three geomagnetic superstorms occurring on 29--30 October 2003, 30--31 October 2003, and 20--22 November 2003. The corresponding density enhancements peaked around 400%, 500%, and 800% of the quiet-time values in both noon and midnight sectors. The disturbances showed strong noon--midnight and hemispheric/seasonal asymmetry. In the noon sector, the average density enhancement was stronger in summer (southern) than in the winter (northern) hemisphere. In the midnight sector, however, no general conclusion can be drawn about the seasonal effect. Stronger density enhancements occurred in the summer hemisphere during the second and third storm events, while in the winter hemisphere during the first storm event. The relative intensity of the disturbance between day and night is strongly dependent on our definition of the intensity. When expressed in absolute terms (storm-quiet), the density enhancement at night was generally less than half of that at day during all three storms. When expressed in percentage terms ($frac{storm-quiet}{quiet}$), however, the enhancement at night was comparable to or even larger than that at day. The propagation of the disturbance from high to low latitudes during the November storm was faster in summer (southern) than in the winter (northern) hemisphere on both dayside and nightside. In the winter hemisphere, the propagation on the dayside was found to be faster than on the nightside. These variations of the seasonal/hemispheric asymmetry with local time and individual event imply that the many different competing processes involved in a magnetic storm may also vary dramatically with local time and from event to event. The MSIS90 model was unable to reproduce most of the observed features of these three storms.