input: Write a function to create a Python class which represents a 3D Vector. The class should have an initializer to accept x, y, and z coordinates, and a `__str__` method to return the vector in the format "(x, y, z)". Additionally, provide a test case using assert to validate the functionality of the class.
  
  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})"