Pacino has a Bronx accent, mama sounds more like Brooklyn. Robert Duvall is not a native New Yorker, so he probably used a voice coach to develop a sort of New York accent. Brando improvised his, no doubt, some Italian and some New York.

I don't know that Michael's accent changed so much as his voice. Someone can enlighten me here, but I think he was very sick and got that gravelly voice at some point in his life. With the voice change, his diction is more clipped, so it sounds like a different accent, but I don't think it is.