Which one is the most stable canonical structure?And why? According to my book the answer is given that structure III is the most stable because all the atoms have complete octets.

But I think it should be II because of,

(a) Structure III has positive charge on a highly electronegative atom and 

(b) In structure II, the positive charge can be stabilized by the Indicutive effect of Oxygen as well as resonance.

Can anyone provide a more convincing explanation as to why III is more stable?
You're supposed to recognize that oxygen which has more valence electrons than carbon can take on the positive charge without giving up its octet, thus favoring Structure III.

Now, is it really true that the oxygen atom has more positive charge than the formally positive carbons in the other two structures, when oxygen is also more electronegative versus the octet rule?  I suspect that is beyond the scope of your course material.  You have to accept a higher priority for the octet rule in this situation whether that is real or not.

I hate oversimplified textbook problems :-( .