input: Write a function to calculate the maximum, minimum, and mean number of elements in a given string. The function should return a tuple containing the maximum element, the minimum element, and the mean value of all elements in the string. If the string is empty, the function should return (None, None, None).
  
  def get_stats_for_string(string):
      if not string:
          return (None, None, None)
      num_elements = len(string)
      max_element = max(string)
      min_element = min(string)
      mean_element = sum(map(ord, string)) / num_elements
      return (max_element, min_element, mean_element)
  
  assert get_stats_for_string("abcdef") == ('f', 'a', 100.0)
  assert get_stats_for_string("xyz") == ('z', 'x', 120.0)
  assert get_stats_for_string("") == (None, None, None)
  assert get_stats_for_string("abcABC") == ('c', 'A', 65.5)
output: def get_stats_for_string(string):
      if not string:
          return (None, None, None)
      num_elements = len(string)
      max_element = max(string)
      min_element = min(string)
      mean_element = sum(map(ord, string)) / num_elements
      return (max_element, min_element, mean_element)