input: Write a function to create a Python class that represents a 3D Vector with attributes x, y, and z, and a method to return the vector's string representation. If the instance is empty, it should return "(0, 0, 0)" by default.
  class Vector3D:
      def __init__(self, x=0, y=0, z=0):
          self.x = x
          self.y = y
          self.z = z
      
      def __str__(self):
          return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"
  
  assert str(Vector3D(1, 2, 3))=="(1, 2, 3)"
  assert str(Vector3D(-1, -2, -3))=="(-1, -2, -3)"
  assert str(Vector3D())=="(0, 0, 0)"
  assert str(Vector3D(5, 0, 10))=="(5, 0, 10)"
output: 
  class Vector3D:
      def __init__(self, x=0, y=0, z=0):
          self.x = x
          self.y = y
          self.z = z
      
      def __str__(self):
          return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"
  