Sri Lanka’s stock market rose to a three-week high on Friday, as foreign investors bought in weakened shares for the 11th straight session, while the rupee fell on importer dollar demand.    The main share index gained 1.07 percent, or 58.70 points, to close at 5,566.30. It has risen 11 percent over the last seven sessions from an 18-month low, once shares dropped into oversold territory. “Even though the index seems to have found temporary support, we believe that the economy may see signs of hitting a plateau,” Stefan Juriansz, a technical analyst at the Bartleet Religare Securities in Colombo, said.