MEMPHIS, Tenn. — FedEx shares plunged more than three percent early Thursday after the shipping giant beat forecasts with fiscal third-quarter profit of $521 million, or $1.65 a share, up from $231 million, or 73 cents a share a year ago. Revenue in the quarter rose to $10.56 billion from $9.66 billion in the same period last year. Analysts polled by FactSet Research expected the firm to earn $1.34 a share, on revenue of $10.6 billion. But shares slipped at market open, as the company downgraded its US economic growth forecast to 2.1 percent this year, from 2.2 percent previously, and lowered its global growth outlook to 2.3 percent, from 2.9 percent. FedEx CEO Fred Smith said following the results that the company would idle some planes until the US market improved, Dow Jones Newswires reported. Looking forward, FedEx said it expects fourth-quarter earnings of $1.75 to $2.00 a share and fiscal-year earnings of $6.35 to $6.60 a share.