Akm[k_,m_]:=Piecewise[{{0, k != 3 || (m != -3 && m != -2 && m != -1 && m != 0 && m != 1 && m != 2 && m != 3)}, {I*B[3, 3], k == 3 && (m == -3 || m == 3)}, {A[3, 2], k == 3 && (m == -2 || m == 2)}, {I*B[3, 1], k == 3 && (m == -1 || m == 1)}}, A[3, 0]]