(Corrects typo in the second paragraph) SAN FRANCISCO, Nov 16 (Reuters) - A third senator has said he would oppose Republican U.S. President Donald Trump’s controversial pick for the Federal Reserve, former economic adviser Judy Shelton, according to the Washington Post and CNBC. Opposition from Tennessee’s Lamar Alexander, a Republican, would trim the margin of support for the Shelton, but may not prevent her confirmation. Republicans have a 53-47 majority in the current Senate, and even with three Republicans voting against, Vice President Mike Pence could cast a tie-breaking vote. And it may not come to that, should the confirmation vote be held in the next few days: the Washington Post reported that Alexander will not be in Washington this week. (Reporting by Jonnelle Marte, Ann Saphir; Editing by Aurora Ellis)