input: Write a function to calculate the maximum, minimum, and average values from a given list of numbers. If the list is empty, return None for all 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([-1, -2, -3, -4, -5])==(-1, -5, -3.0)
  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