I think that there is also a parallel between the deaths of Mary and Fredo. Remember when Michael confesses, when he describes Fredo's death, he says "I killed my father's son," as if, as a parent, he only now realized the enormity of what he did. It was as if the only fitting punishment for him was to feel the ultimate pain, that of a parent losing a child.
Although GF3 had some very bad moments, the premise of Michael seeking redemption was an interesting one. I loved how he tried to explain his life to Kay, how he he had done his dastardly deeds only to protect his family, just as Vito had. However, he went horribly awry. And I felt that his speech to Don Tommasino at the coffin was quite telling, as I think that not only is he speaking to Tommasino, but to his father as well.
I do feel that the ending of GF2 was far superior, I also feel that GF3 had some great scenes and did touch our hearts, which is a great accomplishment.