input: Write a function to generate a Python class that represents a 3-dimensional vector. The class should have an initialization method to set the x, y, and z coordinates and a method to calculate the length of the vector. If the input coordinates are integers or floating-point numbers, the output should be the corresponding class instance.
  
  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
  assert Vector3D(2.5, 3.5, 4.5).length() == 6.304837927755599
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