input: Write a function to construct a class in Python that implements two methods to add two numbers and add three numbers respectively. Return the sum from each method. If the number of arguments is not as expected, 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 "Error: Arguments 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 "Error: Arguments must be numbers."
  
  assert Calculator().add_two_numbers(1, 2)==3
  assert Calculator().add_two_numbers(5.5, 2.3)==7.8
  assert Calculator().add_three_numbers(1, 2, 3)==6
  assert Calculator().add_three_numbers(5.5, 2.3, 1.2)==9.0
  assert Calculator().add_two_numbers("1", 2)=="Error: Arguments must be numbers."
  assert Calculator().add_three_numbers(1, 2, "3")=="Error: Arguments 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 "Error: Arguments 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 "Error: Arguments must be numbers."