* Wheat firms for third session, set for second week of gains * Chicago corn futures ease on pressure from U.S. harvest (Adds quote, IGC forecast, fund positioning) SINGAPORE, Sept 24 (Reuters) - Chicago wheat futures rose for a third straight session on Friday and headed for their second weekly gain, driven by concerns about tighter global supplies due to adverse weather conditions. Corn lost ground on U.S. harvest pressure, while soybeans edged higher. “U.S. futures and spot prices made larger gains again,” said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia. “To some extent, we see that as a ‘marker’ that flags greater investor participation in the rises.” The most-active wheat contract on the Chicago Board Of Trade (CBOT) added 0.1% at $7.18-1/4 a bushel by 0243 GMT, having closed up 1.7% on Thursday. Wheat is up nearly 1.5% for the week. For the week, soybeans were little changed, while corn was largely flat. Last week, soybeans ended nearly flat and corn gained 1.8%. Wheat production in Russia, Canada and the European Union is set to take a hit due to abnormal weather conditions.Soybean export sales last week totalled 912,900 tonnes, near the high end of market forecasts that ranged from 500,000 to 1.1 million tonnes, according to the U.S. Department of Agriculture. Export sales of 373,000 tonnes of corn were near the low end of trade expectations for 300,000 to 800,000 tonnes. The International Grains Council on Thursday raised its forecast for the 2021/22 global corn crop, reflecting an improved outlook for the United States and Ukraine. In its monthly update, the inter-governmental body increased its 2021/22 world corn (maize) crop outlook by 7 million tonnes to a record 1.209 billion tonnes. Commodity funds were net buyers of CBOT wheat, corn, soyoil and soybean futures contracts on Thursday and net sellers of soymeal futures, traders said. (Reporting by Naveen Thukral; Editing by Ramakrishnan M.)