a = [
  [1,2,3,4,5,6,7,8,9],
  [1,2,3,4,5,6,7,8,9],
  [1,2,3,4,5,6,7,8,9],
  [1,2,3,4,5,6,7,8,9],
  [1,2,3,4,5,6,7,8,9],
  [1,2,3,4,5,6,7,8,9],
  [1,2,3,4,5,6,7,8,9],
  [1,2,3,4,5,6,7,8,9],
  [1,2,3,4,5,6,7,8,9]
]

b = [[x for r in a[i:i+3] for x in r[j:j+3]] for j in range(0,9,3) for i in range(0,9,3)]

print(b)