input: Write a function to create a class named `Calculator` with two methods: `add_two_numbers(self, a, b)` which returns the sum of two numbers and `add(self, a, b, c)` which returns the sum of three numbers. Ensure that the class and methods are correctly implemented and provide test cases to validate the functionality.
  
  
  class Calculator:
      def __init__(self):
          pass
  
      def add_two_numbers(self, a, b):
          return a + b
  
      def add(self, a, b, c):
          return a + b + c
  
  calculator = Calculator()
  result1 = calculator.add_two_numbers(1, 2)
  assert result1 == 3
  result2 = calculator.add(1, 2, 3)
  assert result2 == 6
  
output: class Calculator:
      def __init__(self):
          pass
  
      def add_two_numbers(self, a, b):
          return a + b
  
      def add(self, a, b, c):
          return a + b + c