Complex sentence: Margaron commanded the cavalry which were all organized as provisional regiments, the 3rd, 4th and 5th Dragoons and the 1st Chasseurs à Cheval, plus 100 volunteers.
Split into simpler sentences: Margaron commanded the cavalry, which were all organized as provisional regiments. They were the 3rd, 4th and 5th Dragoons and the 1st Chasseurs à Cheval, plus 100 volunteers.