The word "salvo" is often related to an unpredictable event, i.e. " you' ll arrive by train to your town except (salvo) a strike"
About "eccetto" and "tranne", they are related to situations or actions that happen not in free way, i.e. " you can eat everything in this restaurant except (eccetto/tranne) fish because it is finished" or in not completely way, i.e. "the forecasts forecast a bad weather in Italy except (eccetto/tranne) some regions".
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