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, minimum element, and the mean of the elements. Assume the string contains only numeric characters. If the string is empty, 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([int(char) for char in string]) / num_elements
      return (max_element, min_element, mean_element)
  
  assert get_stats_for_string("12345") == ('5', '1', 3.0)
  assert get_stats_for_string("543") == ('5', '3', 4.0)
  assert get_stats_for_string("") == (None, None, None)
  assert get_stats_for_string("111") == ('1', '1', 1.0)
  assert get_stats_for_string("987654321") == ('9', '1', 5.0)
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([int(char) for char in string]) / num_elements
      return (max_element, min_element, mean_element)