If Clauswitz speaks of 3-rank square "according to the old method", probably this means he wrote after 1810, when a new form of square was introduced in the Prussian army: a kind of closed column, with all ranks standing.

The latest "old method" is described in the Prussian 1798 instrcution for the infantry. There, in the 4th, chapter, "Of firing.", it is written (an ancient English translation, transcription made by John Cook):

That young Officers may not be led away by false ideas on this matter, it is necessary to mention, that when a battalion finds itself hard pressed by the enemy's cavalry whether it is in square or in any other situation, the best defence that can be adopted is, for the front rank to kneel, cock their firelocks and placing the butts firmly on the ground, whilst the centre and rear ranks only fire. In this situation should the fault be committed of firing all together, still the front rank will form a respectable reserve whose bayonets and fire will certainly stop the enemy.