input: Write a function to create a Python class representing a 3-dimensional vector with methods to calculate the length of the vector. Include the `__init__` method to initialize the vector's coordinates and a `length` method to compute the vector's length.
  
  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
  
  assert Vector3D(3, 4, 5).length() == 7.0710678118654755
  assert Vector3D(1, 1, 1).length() == 1.7320508075688772
  assert Vector3D(0, 0, 0).length() == 0.0
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