input: Write a function to create a class called Calculator with methods to add two numbers and three numbers. The class should have an initialization method and two addition methods. If the provided numbers are valid, the method should return the sum, otherwise, it should return an error message.
  assert Calculator().add_two_numbers(1, 2) == 3
  assert Calculator().add_two_numbers(-1, 1) == 0
  assert Calculator().add(1, 2, 3) == 6
  assert Calculator().add(-1, -1, 2) == 0
output: 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
          else:
              return "Error: Invalid input"
  
      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
          else:
              return "Error: Invalid input"