input: Write a function to move the last 3 elements of a list to the beginning of the list.
  assert move_last_3([1, 2, 3, 4, 5, 6])==[4, 5, 6, 1, 2, 3]
  assert move_last_3([10, 20, 30, 40, 50, 60, 70])==[50, 60, 70, 10, 20, 30, 40]
  assert move_last_3([5, 9, 14, 23, 42, 56])==[23, 42, 56, 5, 9, 14]
output: def move_last_3(lst):
      return lst[-3:] + lst[:-3]