GRAINS-Corn slips from three-week high; wheat and soy edge higher

(Updates prices, adds quotes, changes byline, changes headline, changes dateline from previous PARIS/KUALA LUMPUR ) By Cassandra Garrison MEXICO CITY, March 27 (Reuters) – Chicago corn futures were down on Monday, retreating from a more than three-week high in the previous session, and wheat edged higher on earlier short covering and uncertainty over Black Sea exports. Soybeans edged higher, supported by a jump in soybean oil prices. Private exporters reported the sale of 112,800 tonnes of corn to unknown destinations, the U.S. Agriculture Department said. After last week’s heavy Chinese demand, corn was slipping as traders speculated on weather conditions…