I agree with you. Part II was every bit the equal of I.
The first time I saw Part III, it was just awful. A couple of years later I saw it again and it still is not very good but not quite as bad as it seemed at first.
I bet “The Godfather” has more lines which are regularly quoted than just about any movie ever.
3 had could have been palatable if it could have risen to a percentage of the greatness of 1 & 2. It seemed to me as trying to drag along on the coattails of the previous.