CHICAGO, Feb 22 (Reuters) - U.S. corn and soybean futures fell to three-year lows on Thursday, dragged down by expectations of ample South American harvests and rising stockpiles at a time of uncertain demand, analysts said.

Wheat futures ended mostly higher after a choppy session, drawing support from new U.S. sanctions expected against top global wheat exporter Russia that could affect trade flows, as well as inter-market spreading against corn and soy.

Chicago Board of Trade (CBOT) March corn futures settled down 5 cents at $4.06 per bushel after sliding to $4.04-1/4, the lowest on a continuous chart of the most-active corn contract since November 2020.

CBOT March soybeans ended down 13 cents at $11.47-3/4 a bushel after hitting $11.46-3/4, the lowest for a most-active soybean contract since December 2020. Benchmark CBOT May wheat rose 1-1/4 cents to finish at $5.79-1/4 a bushel.

Declines in CBOT corn and soybean futures have accelerated since the start of 2024 as South American harvest prospects have brightened, despite periods of stressful weather.

"The crops in South America look fairly good. It's still an unknown how large the Brazilian corn crop will end up being, but the weather is promoting development right now," said Terry Reilly, senior agricultural strategist with Marex.

Argentina's corn and soybeans are continuing to improve due to recent moisture, the Buenos Aires grains exchange said on Thursday, with more rain expected in coming days after wet weather helped curb crop damage from a January heat wave.

Argentina's Rosario grains exchange cut its estimates this week for the country's 2023/24 soybean and corn harvests, but the 49.5 million metric tons of soy and 57 million tons of corn are still a huge improvement on the previous year.

Following a record-large U.S. corn harvest in 2023, outlooks for rising grain stockpiles and falling Chinese demand for animal feed have spurred speculators to build massive net short positions in corn and soybean futures.

The sell-off could continue, said Ole Houe at IKON Commodities in Sydney.

"With the weight of the South American crop now coming at us, I think we'll go below $4," he said, though he warned that the huge short position held by investors meant that when a rally does occur, it is likely to be rapid.

(Reporting by Julie Ingwersen in Chicago; Additional reporting by Peter Hobson in Canberra and Sybille de La Hamaide in Paris Editing by Matthew Lewis and Sam Holmes)