Sforno on Exodus 14:7/ספורנו על שמות י״ד:ז׳

1וכל רכב מצרים, the chariots of average quality of which there were many.

2ושלישים על כלו, Pharaoh appointed experienced officers even for the chariots of this second cadre. שלישים are officers who have battle experience to their credit. The aggressive ability of any army depends on its officer corps and the intelligence of the generals to devise schemes and battle plans which will lead to victory.