input: Write a function to create a Python class which represents a 3D Vector. The class should have an initializer that takes three parameters (x, y, z) and a string representation method that returns the vector in the format "(x, y, z)". Use the following assert statements to verify the functionality of the class.
  assert str(Vector3D(1, 2, 3))=="(1, 2, 3)"
  assert str(Vector3D(0, 0, 0))=="(0, 0, 0)"
  assert str(Vector3D(-1, 2, -3))=="(-1, 2, -3)"
output: class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
  
      def __str__(self):
          return f"({self.x}, {self.y}, {self.z})"