Man, that would have been some fucked up movie if any combination of those scenarios took shape. GF III is not a bad movie, it's just not quite up to the par of the previous two and that is because of a plot that wasn't as tight as the other two (although it was more ambitious and very adventurous), Sofia's acting and the absence of Robert Duvall, which seriously diminshed the number of recognizable characters in the film to Kay, Neri, Tomasino, Connie and Michael. I do think they made the best of what they had and as DBCH pointed out, yes it could have been alot worse.

I give GF III 7.8 out of 10. It would have perhaps been as high as 9.0, maybe even higher if it lacked Sofia Coppola, had Robert Duvall, lacked Agent Orange, had a little more plot development on the character development of Vincent.

I give GF II 9.3 out of 10 and the Gofather 9.5 out of 10. GF I gets a better grade because I feel it's a bit more concise although GF II is a very awesome movie all the same.