input: Write a function to implement a class named Calculator which contains two methods to add two numbers and another method to add three numbers. The first method should be named add_two_numbers and take two parameters, while the second method should be named add and take three parameters. If the input is valid, the function should return the sum of the numbers provided.
  
  assert Calculator().add_two_numbers(1, 2) == 3
  assert Calculator().add(1, 2, 3) == 6
  assert Calculator().add_two_numbers(-1, -2) == -3
  assert Calculator().add(-1, -2, -3) == -6
  
output: class Calculator:
      def add_two_numbers(self, a, b):
          return a + b
      
      def add(self, a, b, c):
          return a + b + c