* Soybean futures gain more than 12% in three weeks of rally * Strong Chinese demand, dry weather in South America support (Adds quote in paragraph 3; details on Argentine weather, fund positioning) SINGAPORE, Nov 20 (Reuters) - Chicago soybeans rose for a sixth consecutive session on Friday, with the market poised for a third weekly gain, as robust Chinese demand and dry South American weather supported prices. Corn rose from Thursday’s weak close while wheat edged higher. “There are increasing supply concerns and feed demand is really strong as China is rebuilding its pig herd,” said Phin Ziebell, an agribusiness economist at National Australia Bank. The most-active soybean contract on the Chicago Board Of Trade (CBOT) has gained around 3% this week, taking the gain in three weeks to more than 12%. Soybeans up 0.7% at $11.86 a bushel by 0228 GMT. Earlier this week, soybeans hit a June 2016 high of $11.90 a bushel. Corn has added more than 3% for the week, also poised for a third consecutive weekly gain and wheat was trading little changed for the week. Weekly U.S. Department of Agriculture (USDA) export data on Thursday showed soybean export sales last week at a low for the 2020/21 marketing year, but total sales were still above trade expectations at 1.388 million tonnes. Corn export sales were also slightly above expectations, while wheat sales were below forecasts. Argentine soy planting jumped 16 percentage points over the last week, propelled by rains that brought relief to key drought-hit areas, the Buenos Aires Grains Exchange said on Thursday, although many parts of the country remained woefully dry. Brazil, the world’s biggest soybean exporter, is also facing dry weather. Commodity funds were net buyers of Chicago Board of Trade soybean and soyoil futures contracts on Thursday and net sellers of corn, wheat and soymeal, traders said. (Reporting by Naveen Thukral, Editing by Sherry Jacob-Phillips)