input: Write a function to generate a Python class to represent a 3-dimensional vector with an initialization method and a method to calculate the length of the vector. The class should take three arguments for the x, y, and z components of the vector.
  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
  assert Vector3D(3, 4, 5).length() == (3**2 + 4**2 + 5**2)**0.5
  assert Vector3D(1, 1, 1).length() == (1**2 + 1**2 + 1**2)**0.5
  assert Vector3D(0, 0, 0).length() == (0**2 + 0**2 + 0**2)**0.5
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