The second term is equal to C because I3 is the standard way of expressing the 3×3 identity matrix and when multiplied by C just reproduces C (by definition of identity). AT is the transposed version of A (columns and rows interchanged), and should not be confused with A-1 (the inverse).
If the elements of A are represented by aij, then the elements of AT are represented by aji. The matrix product has diagonal symmetry and is, of course, another 3×3 matrix.