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No, Sofia's wasn't the worst performance in GFIII (IMO, that dubious title belongs either to Eli Wallach or George Hamilton), but it was far from acceptable. The real point is that GF and II had inspired casting and no weak performances. That's one of the reasons why GFIII, though not a terrible film overall, is nowhere near the same league as GF and II. It is, IMO, an acceptable denouement to the Godfather Saga, but an unworthy successor to the first two films.