By Marc Jones LONDON (Reuters) - European shares turned down and the dollar slipped from a three-year high on Wednesday after weak data from China and a rating downgrade of Italy deepened caution pending the Federal Reserve's latest minutes. Europe's broad FTSEurofirst 300 buckled after a positive start to be down 0.3 percent by mid-morning, as data showing China's exports fell for the first time in 17 months was followed by soft manufacturing figures from France, the Netherlands and Greece. ...