* Corn futures hit lowest levels since December 2020

* USDA crop reports show brisk harvest, little change to crop quality

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 as the Midwest harvest expanded, adding to fears of a supply glut at a time of weak export demand for U.S. supplies, traders said.

The benchmark CBOT December corn settled down 4-3/4 cents, about 1%, at $4.71-1/2 per bushel, after hitting $4.69, the lowest price on a continuous chart of the most-active contract since December 2020.

"It's simply the lack of export demand for corn. I think it's probably overdue for this kind of sell-off," said Mark Soderberg, a senior agriculture market analyst with ADM Investor Services in Chicago.

"We continue to be undercut by Brazil in the global marketplace. And domestically, demand is strong for ethanol production, but that's just not enough to make up for it," Soderberg added.

After the CBOT close, the U.S. Department of Agriculture (USDA) said the U.S. corn harvest was 9% complete, ahead of the five-year average of 7% but just behind an average of analyst estimates for 10%. The USDA rated 51% of the crop as good to excellent, a percentage point lower than a week earlier.

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.

Soybeans and wheat fell on Monday, following the weaker trend.

CBOT November soybeans settled down 23-1/2 cents at $13.16-3/4 a bushel after falling to $13.15-1/2, the contract's lowest since Aug. 16.

Soderberg cited a smaller-than-expected August U.S. soy crush figure released on Friday as souring investor sentiment, saying, "The domestic usage was really driving soybean demand, and last month, August, it was disappointing."

After the CBOT close, the USDA said the U.S. soybean harvest was 5% complete, ahead of the five-year average of 4% and the average analyst estimate, also 4%. The USDA rated 52% of the crop as good to excellent, unchanged from a week ago.

CBOT wheat prices fell more than 2% on Monday, with the December contract settling down 13 cents at $5.91-1/4 a bushel, as large supplies from Russia continued to flow onto global markets.

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