input: Write a function to create a Python class that represents a 3D Vector. The class should have an initializer that takes in the x, y, and z components of the vector, and a method to return the vector as a string in the format "(x, y, z)". The function should return the class definition.
  
  assert Vector3D(1, 2, 3).__str__() == "(1, 2, 3)"
  assert Vector3D(0, 0, 0).__str__() == "(0, 0, 0)"
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})"