On an Eisenstein Series of Degree 3

Let T_n Denote the modular group of degree n, namely the group of 2n-rowed integral square matrices with n-rowed A, B, C, D satisfying the conditions AD' - BC' = E_n (the n-rowed identity matrix), AB' = BA' and CD'=DC', where, for any matrix P, its transpose is denoted by P'.