input: Write a function to create a Python class representing a 3D Vector with an initializer that takes three parameters x, y, and z, and a string representation method that returns the vector in the format "(x, y, z)". The function should return the class definition.
  assert create_3d_vector_class()==Vector3D
  vector = Vector3D(1, 2, 3)
  assert str(vector)=="(1, 2, 3)"
  assert vector.x==1
  assert vector.y==2
  assert vector.z==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})"