input: Write a function to construct a class in Python which implements two methods to add two numbers and another method to add three numbers. The function should define the class structure as shown and return an instance of the class.
  
  class Calculator:
   def __init__(self):
     pass
      
   def add_two_numbers(self, a, b):
     return a + b
  
   def add_three_numbers(self, a, b, c):
     return a + b + c
  
  assert res.add_two_numbers(1, 2) == 3
  assert res.add_three_numbers(1, 2, 3) == 6
  assert res.add_two_numbers(0, 0) == 0
  assert res.add_three_numbers(-1, -1, -1) == -3
output: class Calculator:
      def __init__(self):
          pass
      
      def add_two_numbers(self, a, b):
          return a + b
  
      def add_three_numbers(self, a, b, c):
          return a + b + c