Part 3 of the second Trilogy "The Strong Sylow Theorem for the Prime p in the Locally Finite Classical Groups" & "The Strong Sylow Theorem for the Prime p in Locally Finite and p-Soluble Groups" & "Augustin-Louis Cauchy's and Évariste Galois' Contributions to Sylow Theory in Finite Groups" proves for a subgroup G of the finite group H Lagrange's theorem and three group theorems by Cauchy, where the second and the third were concealed, by a unified method of proof consisting in smart arranging the elements of H resp. the cosets of G in H in a rectangle/tableau. Cauchy's third theorem requires the existence of a Sylow p-subgroup of H. These classical proofs are supplemented by modern proofs based on cosets resp. double cosets which take only a few lines. We then analyse first his well-known published group theorem of 1845/1846, for which he constructs a Sylow p-subgroup of Sn, thereby correcting a misunderstanding in the literature and introducing wreath products, and second his published group theorem of 1812/1815, which is related to theorems of Lagrange, Vandermonde and Ruffini. Subsequently we present what Galois knew about Cauchy's group theorems and about Sylow's theorems by referring to his published papers and as well to his posthumously published papers and to his manuscripts. We close with a detailed narrative of early group theory and early Sylow theory in finite groups.
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