(New throughout; updates prices, adds quotes, changes byline, changes dateline from previous PARIS/CANBERRA)

CHICAGO, Feb 21 (Reuters) - U.S. corn futures fell to three-year lows on Wednesday, inching closer to the $4-per-bushel mark as ample domestic grain supplies and strong South American crop prospects weighed on sentiment, analysts said.

Soybeans and wheat followed the lower trend, retreating after rallies a day earlier.

As of 12:53 p.m. CST (1853 GMT), Chicago Board of Trade March corn was down 7-1/2 cents at $4.11-1/4 per bushel after hitting $4.11, the lowest since November 2020 on a continuous chart of the most-active corn contract.

CBOT March soybeans were down 14-1/4 cents at $11.64-3/4 a bushel and most-active May wheat was down 3-1/2 cents at $5.75-3/4 a bushel.

Corn posted the biggest declines on a percentage basis. Some analysts attributed the sell-off to commodity funds adding to their already-large net short position, while others cited selling by farmers unloading grain stored from the 2023 harvest.

"Guys have been waiting for higher prices that haven't come," said Ted Seifried, chief market strategist for the Zaner Group. "Hope is waning and time is ticking. The farmer is feeling stressed right now, and one of the ways to alleviate that stress is to let the bushels go."

Soybeans followed the weaker trend as the harvest progressed in Brazil, even as analysts debated the size of South American crops.

"The bottom line is that our demand is hurting, and the market keeps telling us we've got enough beans out there," said Tom Fritz, a partner with EFG Group in Chicago.

Traders continue to watch for signs of renewed demand from China following last week's Lunar New Year holidays and as the government tries to revive a property sector that has cast doubts over China's wider economy.

Meanwhile, expected rainfall over the next few days in Argentina's Pampas region will likely boost the 2023/24 soybean and corn crops, the Buenos Aires grains exchange said on Wednesday.

By last Thursday, 32% of Brazil's planted soybean area had been harvested, according to consultants AgRural, up 9% from a week earlier and ahead of last year's pace. (Reporting by Julie Ingwersen; additional reporting by Gus Trompiz in Paris and Peter Hobson in Canberra; editing by Jonathan Oatis)