Hi,

When you combust a molecule, you try to bring it to its most fully oxidized state, forming as many CO bonds as possible, which are comparatively stronger because you forming a C=O and getting rid of C-H bonds which are weaker.

When you already have C=O bonds in your molecule, you cannot release as much energy because your molecule is already partially oxidized. So the molecule that is least oxidized (III) releases the most energy.

This makes sense because fats which are mostly CH bonds contain much more energy per gram than sugars which have a combination of C=O, CH and CO bonds.

Hope this helps!