Eastman Kodak of Rochester, N.Y., reported an 8% sales increase and 7% improvement in net earnings for the third quarter of 1986. Worldwide sales for the quarter also increased, the company said, reflecting volume gains and the favorable effects of strengthening foreign currencies. "Gains in sales volume demonstrate that our business is growing. At the same time, the benefits of our on-going cost-improvement program are increasingly evident," a spokesman said. "We look forward to continuing growth in sales and solid gains in earnings from operations, this year, in 1987 and beyond."