input: Write a function to calculate the maximum, minimum, and average values from a given sequence of numbers. The function should return these values in that order. If the input list is empty, it should return None for all three values.
  
  def calculate_metrics(list):
      if not list:
          return None, None, None
      maximum = max(list)
      minimum = min(list)
      n = len(list)
      total = sum(list)
      average = total / n
      return maximum, minimum, average
  
  assert calculate_metrics([3, 4, 5, 6, 21, 8]) == (21, 3, 7.833333333333333)
  assert calculate_metrics([10, 10, 10]) == (10, 10, 10.0)
  assert calculate_metrics([]) == (None, None, None)
output: def calculate_metrics(list):
      if not list:
          return None, None, None
      maximum = max(list)
      minimum = min(list)
      n = len(list)
      total = sum(list)
      average = total / n
      return maximum, minimum, average