input: from typing import List
  
  def find_maximum(numbers: List[int]) -> int:
      """ Find the maximum number from a list of three integers.
      >>> find_maximum([3, 4, 5])
      5
      >>> find_maximum([10, 20, 30])
      30
      """
output:     if len(numbers) != 3:
          raise ValueError("The list must contain exactly three integers.")
      return max(numbers)