input: Write a function to create a class named Calculator with two methods to add two numbers and three numbers respectively. If the given numbers are valid, return their sum, else return an error message.
  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 "Invalid input: both a and b must be numbers."
  
      def add_three_numbers(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 "Invalid input: a, b, and c must be numbers."
  
  calculator = Calculator()
  assert calculator.add_two_numbers(1, 2) == 3
  assert calculator.add_three_numbers(1, 2, 3) == 6
  assert calculator.add_two_numbers(1.5, 2.5) == 4.0
  assert calculator.add_three_numbers(1.5, 2.5, 3.5) == 7.5
  assert calculator.add_two_numbers('1', 2) == "Invalid input: both a and b must be numbers."
  assert calculator.add_three_numbers(1, 2, '3') == "Invalid input: a, b, and c must be numbers."
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
      else:
          return "Invalid input: both a and b must be numbers."
  
  def add_three_numbers(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 "Invalid input: a, b, and c must be numbers."