input: 
  def create_3d_vector_class() -> str:
      """ Create a Python class which represents a 3D Vector and return a string of the class definition.
      >>> create_3d_vector_class()
      'class Vector3D:\n    def __init__(self, x, y, z):\n        self.x = x\n        self.y = y\n        self.z = z\n    \n    def __str__(self):\n        return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"'
      """
      class_str = """class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def __str__(self):
          return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"
  """
      return class_str
  
output:     class_str = """class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def __str__(self):
          return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"
  """
      return class_str