* Corn futures hit lowest levels since December 2020

* Soybean, wheat futures follow weak trend

NEW YORK, Sept 18 (Reuters) - U.S. corn futures on the Chicago Board of Trade (CBOT) hit their lowest levels in nearly three years on Monday on pressure from the expanding U.S. harvest and export competition from Brazilian supplies, traders said.

Soybeans and wheat followed the weaker trend.

As of 12:02 p.m. CDT (1702 GMT), CBOT December corn was down 5-3/4 cents at $4.70-1/2 per bushel after dipping to $4.69, the lowest on a continuous chart of the most-active corn contract since December 2020.

Technical selling in December corn accelerated as the contract dropped below chart support at $4.73-1/2, a so-called "double bottom" on the contract's chart that was hit on Aug. 16 and again on Sept. 12.

"On December corn, if we close below $4.72, it starts looking bad," said Mark Schultz, chief analyst at Minneapolis-based Northstar Commodity. "A close below that will look bearish."

Meanwhile, clear skies across much of the U.S. Farm Belt allowed combines to roll, with the harvest of corn and soybeans still in the early stages. The U.S. Department of Agriculture (USDA) is scheduled to report on U.S. corn and soybean harvest progress in a weekly report due later on Monday.

Market players are watching for signs that a hot, dry summer might have crimped crop yields below the USDA's latest estimates. The government trimmed its projections of the average U.S. corn and soybean yield in a monthly report last week.

"There's not much there, weather-wise," Schultz said. "As far as harvest goes, we're not running into consistent, devastating yields."

Demand remains a concern. With huge shipments from Brazil reducing overseas demand for U.S. corn, analysts believe stockpiles could reach their highest levels in a decade. CBOT corn prices have fallen 30% this year.

CBOT wheat fell more than 2%, pressured by adequate global supplies and strong export competition from Russian supplies. As of 12:05 p.m. CDT (1705 GMT), CBOT December wheat was down 14-1/4 cents at $5.90 a bushel.

Wheat prices drew underlying support from reduced harvest potential in Australia and Argentina. A spring heat wave across large parts of Australia's southeast, including Sydney, will intensify on Monday.

CBOT November soybeans were down 23 cents at $13.17-1/2 per bushel, shaking off support from fresh U.S. export sales. The USDA confirmed private sales of 123,000 tonnes of U.S. soybeans to China.

(Reporting by Zachary Goelman in New York City; Editing by Will Dunham)