I doubt that Neri or anyone else was Michael's underboss in GFIII because by that time it seemed that he had thrown off most of the operating aspects of running a Mafia family. Though he claimed to be "legitimate" by that time, he was still an eminence grise in the Mafia--apparently serving on the Commission and generally acting as an elder statesman. But most, perhaps all, of his day-to-day activities were in the legitimate world. As he said, Zasa held the "olive oil business," so I don't think he had a need for an underboss. Neri, IMO, was just a bodyguard and general factotem.