In order to make GFIII watchable, someone would have to go back in time and kill Sophia Coppolla's mother the night before Sophia was conceived.
What a
terrible thing to say. Even in jest.
That aside, with all the obvious casting flaws, the main problem w/ GFIII is and always will be the writing, as the preceding parade of 'weak' (not to mention ridiculous) lines has proven.