input: Write a function to implement a class in Python which defines two methods for adding two numbers. The class should have a constructor that initializes the instance. The first method should add two numbers and return the result. The second method should add three numbers and return the result. If the input numbers are not provided, the methods should raise an exception.
  
  class Calculator:
      def __init__(self):
          pass
  
      def add_two_numbers(self, a, b):
          if a is None or b is None:
              raise ValueError("Both numbers must be provided")
          return a + b
  
      def add_three_numbers(self, a, b, c):
          if a is None or b is None or c is None:
              raise ValueError("All three numbers must be provided")
          return a + b + c
  
  calculator = Calculator()
  assert calculator.add_two_numbers(1, 2) == 3
  assert calculator.add_three_numbers(1, 2, 3) == 6
  assert calculator.add_two_numbers(0, 0) == 0
  assert calculator.add_three_numbers(1, 1, 1) == 3
output: 
  class Calculator:
      def __init__(self):
          pass
  
      def add_two_numbers(self, a, b):
          if a is None or b is None:
              raise ValueError("Both numbers must be provided")
          return a + b
  
      def add_three_numbers(self, a, b, c):
          if a is None or b is None or c is None:
              raise ValueError("All three numbers must be provided")
          return a + b + c
  