input: Write a function to create a Python class that represents a 3D Vector with attributes x, y, and z, and includes a method to return the string representation of the vector in the format "(x, y, z)". The class should initialize with three parameters x, y, and z.
  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) + ")"
  assert str(Vector3D(1, 2, 3))=="(1, 2, 3)"
  assert str(Vector3D(-1, 0, 1))=="(-1, 0, 1)"
  assert str(Vector3D(0, 0, 0))=="(0, 0, 0)"
  assert str(Vector3D(10, -5, 2))=="(10, -5, 2)"
  assert str(Vector3D(-12, 3.5, -4)) == "(-12, 3.5, -4)"
output: 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) + ")"