input: Write a function to implement a simple calculator class with two methods to add two numbers. The first method should add two integers, and the second method should add three integers. If the given input is not provided correctly, return None.
  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
  
  assert Calculator().add_two_numbers(1, 2)==3
  assert Calculator().add(1, 2, 3)==6
  assert Calculator().add_two_numbers(0, 0)==0
  assert Calculator().add(0, 0, 0)==0
  assert Calculator().add_two_numbers(-1, 1)==0
  assert Calculator().add(-1, -1, -1)==-3
output: 
  class Calculator:
   def __init__(self):
     pass
      
   def add_two_numbers(self, a, b):
     if isinstance(a, int) and isinstance(b, int):
       return a + b
     return None
  
   def add(self, a, b, c):
     if isinstance(a, int) and isinstance(b, int) and isinstance(c, int):
       return a + b + c
     return None
  