input: Write a function to generate a Python class that represents a 3-dimensional vector. The class should have an initializer that takes three parameters (x, y, z) to set the vector components, and a method to calculate the length of the vector. If the given input is valid, the class should be returned; otherwise, return None.
  assert Vector3D(2, 3, 6).length() == 7.0
  assert Vector3D(0, 0, 0).length() == 0.0
  assert Vector3D(-1, -1, -1).length() == (3**0.5)
  assert Vector3D(1, 2, 2).length() == 3.0
  assert Vector3D(3, 4, 0).length() == 5.0
output: class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def length(self):
          return (self.x**2 + self.y**2 + self.z**2)**0.5