Also, the idea that the division of society into nobles, priests and commoners reflects some ancient Indo-European division is, to put it politely, dubious. Even if the Indo-Europeans themselves had such a division (and the evidence for this is often quite tenuous*), many of the intermediate societies didn't,** and it's more likely that the mediaeval system (which wasn't universal anyway***) was an independent development and had nothing to do with Indo-Europeans.
* For example, Dumezil, one of the main historical proponents of the theory, used the Roman legend that Romulus had established three tribes as evidence. Three divisions of society, three tribes, QED. Except that none of the sources give any indication that these tribes were based on occupation or social class, and the fact that each of them was supposedly responsible for providing soldiers rather suggests otherwise.
** E.g., the Romans didn't (or at least didn't originally) have separate classes of warriors, priests, and farmers. Indeed, their ideal was that of the citizen-soldier, a farmer who took up arms in times of war and then returned to his plough once the fighting was over. Priests, meanwhile, were elected like politicians, and indeed lots of politicians were priests as well (Julius Caesar was pontifex maximus, for example). Similarly, the classical Greeks didn't have a separate warrior class, but drew their soldiers from the ranks of the wealthy farmers who fought as heavy infantry militia.
*** As AE said, the Swedes had a four-estate system. England, meanwhile, had a two-estate system, Lords and Commons, which is still reflected to this day in the organisation of the British Parliament.