Which one is the most stable canonical structure?And why?
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 :-( .