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 three values.
  assert calculate_metrics([3, 4, 5, 6, 21, 8])==(21, 3, 8.166666666666666)
  assert calculate_metrics([10, -5, 25, -15, 0])==(25, -15, 3.0)
  assert calculate_metrics([])==(None, None, None)
output: def calculate_metrics(numbers):
      if not numbers:
          return None, None, None
      maximum = max(numbers)
      minimum = min(numbers)
      average = sum(numbers) / len(numbers)
      return maximum, minimum, average