input: Write a function to create a class in Python that implements two methods to add two numbers. The first method should take two numbers as input and return their sum. The second method should take three numbers as input and return their sum. If the input is invalid, return None.
  Construction of a class to add two numbers is done as shown:
  class Calculator:
   def __init__(self):
     pass
      
   def add_two_numbers(self, a, b):
     if isinstance(a, (int, float)) and isinstance(b, (int, float)):
       return a + b
     return None
  
   def add(self, a, b, c):
     if isinstance(a, (int, float)) and isinstance(b, (int, float)) and isinstance(c, (int, float)):
       return a + b + c
     return None
  
  assert Calculator().add_two_numbers(1, 2) == 3
  assert Calculator().add(1, 2, 3) == 6
  assert Calculator().add_two_numbers(1.5, 2.5) == 4.0
  assert Calculator().add(1.5, 2.5, 3.5) == 7.5
  assert Calculator().add_two_numbers('a', 2) == None
  assert Calculator().add(1, 2, 'c') == None
output: 
  def __init__(self):
      pass
      
  def add_two_numbers(self, a, b):
      if isinstance(a, (int, float)) and isinstance(b, (int, float)):
          return a + b
      return None
  
  def add(self, a, b, c):
      if isinstance(a, (int, float)) and isinstance(b, (int, float)) and isinstance(c, (int, float)):
          return a + b + c
      return None
  