>>> matrix = [
... [1, 1, 1, 0, 0, 0],
... [0, 1, 0, 0, 0, 0],
... [1, 1, 1, 0, 0, 0],
... [0, 0, 2, 4, 4, 0],
... [0, 0, 0, 2, 0, 0],
... [0, 0, 1, 2, 4, 0],
... ]
>>> max( sum([sum(n[0]), n[1][1], sum(n[2])]) for n in [ [ m[y-3:y] for m in matrix[x-3:x] ] for x in range(3,7) for y in range(3,7) ])
19