Transform this 3rd-order ordinary differential equation 
into a set of 3 first-order ODEs:

   y''' - sin(y') + y^3 = 0      (1)

This procedure is covered on page 3 of the ODE notes that I passed out
in class on 14 November.  

First, rewrite the equation to get the highest-order derivative
on the left-hand side, by itself:

   y''' = sin(y') - y^3          (2)

Then define two helper variables, like this:

   z = y'
   w = z'  (= y'')

	(You may name these anything you want; there's nothing 
	 magic about z and w.)

Then rewrite equation (2) using these helper variables, so as to leave
no derivatives on the right-hand side:

   w' = sin(z) - y^3          

...and stick on the equations that you used to define the helper
variables:

   y' = z
   z' = w
   w' = sin(z) - y^3          

Note that it's customary to have the state variables on the left-hand
side of the resulting ODE system.