South Korea's three mobile carriers are estimated to have posted an improved ratio of operating income to total revenue in the third quarter, industry sources said Sunday, as the firms reduced marketing expenditures. The combined sales posted by SK Telecom Co., KT Corp. and LG Uplus Corp. are estimated to have reached 12.5 trillion won in the July-September period, while the operating profit is expected at 1.67 trillion won, the data compiled by industry tracker FnGuide showed.