In the beginning of GF III, Michael says to Kay that the last time they saw eachother was 8 years ago, which would be about 1971. I came to think that this was likely when Tom Hagen died. According to the film's timeline Tom died somewhere in the 1970s so it would fit perfectly. I think it's pretty obvious that both Michael and Kay would attend Tom's funeral so if Tom died after 1971 Michael probably couldn't have made that statement to Kay.