input: Write a function to generate a Python class that represents a 3D Vector with initial coordinates (x, y, z) and provides a string representation of the vector. The class should have an init method to initialize the coordinates and a str method to return the string representation.
  assert Vector3D(1, 2, 3).__str__()=="(1, 2, 3)"
  assert Vector3D(0, 0, 0).__str__()=="(0, 0, 0)"
  assert Vector3D(-1, -2, -3).__str__()=="(-1, -2, -3)"
  assert Vector3D(10, 20, 30).__str__()=="(10, 20, 30)"
  assert Vector3D(1.5, 2.5, 3.5).__str__()=="(1.5, 2.5, 3.5)"
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})"