I watch the entire trilogy almost once per week and I liked part 3, but not as much as the first two. I think they dropped the ball in a number of ways. First off they had a number of characters that had appeared in the first two, but didn't really introduce who they were in part 3. I think they made the same mistake in the part 2 flashbacks (with Tessio and Clemenza), but not to the same extent as part 3. Even Neri's character didn't garner the respect in part 3 that he deserved. Some of the characters that made an appearance in part 3 that were in the previous 2 were:
1. Vincenzo--the love child of Sonny and the woman he was banging on the side (during Connie's wedding) in part 1. Many people do not realize this relationship w/Michael.
2. The Woman that Sonny was banging on the side in part 1/Vincenzo's mom was actually in Part 3--with Vincenzo as they arrived at the reception AND after he bit Zasa's ear off outside of Michael's office.
3. Enzo the Baker--baked the cake for Michael's party in part 3 was the same person that Vito helped to become a US citizen and also helped Michael protect Vito at the hospital in part 1.
4. Michael's "loyal" Sicilian bodyguard (not Fabrizio who planted the bomb that killed Apollonia in pt 1) was in many of the Sicilian scenes in part 3 as well--I believe he was the one that killed Luchese (or one of the others) at the end of pt 3.
5. The hitman that shot Mary, I THINK may have been Michael's bodyguard that betrayed him and planted the bomb that killed Apollonia, but I have been unable to find any definite ID on that yet.
6. Johnny Fontaine-made appearances in all 3, but just appeared to be a cheap wedding singer in pt 3.
7. I thought Eli Wallach played an excellent role as Altobello in part 3 and I wish he would have been introduced in one of the previous ones. Many people don't realized that Eli Wallach is also the same person that played Tuco (or "the Ugly") in the classic spaghetti western "The Good, The Bad and The Ugly" with Clint Eastwood, he also played Mr. Freeze in the old Batman TV shows.

They wouldn't have had to do much to (re-)introduce everyone--maybe just a quick shot of them in their youth 20 years ago. For example, when Michael spoke to Enzo the Baker in part 3, they could have shot back to that part in part 1 when he introduced himself (with flowers in hand) to Michael in the hospital, "...I'm Enzo, I'm here for your father, I help you..."
-Just my two cents worth.