I thought the story arc was complete in GF2. Michael has won the whole world but lost his soul. He's murdered his own brother and irretrievably alienated his wife and stepbrother. I didn't think the story had anywhere to go from there. So even if GF III had been a better movie, it still would have been unnecessary IMO.
GF III is an attempt to show Michael's quest for redemption and forgiveness. To get it, he pays a heavy price -- the death of his daughter. With that in mind, GFIII comletes the arc of the story of Michael's life ending of course with his solitary death, an old and broken man.
