input: Write a function to create a Python class which represents a 3D Vector. The class should have an initializer that takes three arguments (x, y, and z) to set the components of the vector, and a __str__ method to return the vector in the form of "(x, y, z)". If no arguments are provided, the vector should be initialized to (0, 0, 0).
  assert str(Vector3D(1, 2, 3))=="(1, 2, 3)"
  assert str(Vector3D())=="(0, 0, 0)"
  assert str(Vector3D(-1, 0, 1))=="(-1, 0, 1)"
  assert str(Vector3D(5, -5, 5))=="(5, -5, 5)"
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 f"({self.x}, {self.y}, {self.z})"