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. Additionally, it should have a `__str__` method to return a string representation of the vector in the format "(x, y, z)".
  
  assert Vector3D(1, 2, 3).__str__() == "(1, 2, 3)"
  assert Vector3D(0, 0, 0).__str__() == "(0, 0, 0)"
  assert Vector3D(-1, 4, -2).__str__() == "(-1, 4, -2)"
  assert Vector3D(10, 10, 10).__str__() == "(10, 10, 10)"
  assert Vector3D(5, -5, 5).__str__() == "(5, -5, 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})"