im not sure what your trying to show in that mock up.

You can't just apply the rule of thirds to a PORTION of your frame and come out with a balanced composition.  The widescreen frame is quite still quite out of balance.

Also, the brightness and color of objects also need to be taken into account when thinking of composition.  You can apply the rule of thirds all day long, but that big bright moon in the center is a vortex that grabs the eye and will never let go.  To balance the frame, you need to have bright highlights on the dragons as well.  Where is your rim lighting from the moon?  If you didn't do it in your render, then you can paint on a rim light in photoshop.

Gabriel Sabloff
TIRADE STUDIOS