SAO PAULO, Oct 14 (Reuters) - Brazil’s central bank said on Thursday it will offer a currency swap auction of up to $1 billion on Friday, its third consecutive daily sale, as it seeks to prop up the beleaguered real currency. The real is down by around a third against the U.S. dollar this year, with investors spooked by political risks, a fragile economic recovery and the prospect for rising interest rates in more wealthy economies. (Reporting by José de Castro; Editing by Sandra Maler)