CHICAGO, Feb 14 (Reuters) - Soybean and corn futures fell to three-year lows at the Chicago Board of Trade on Wednesday on fund selling and as a three-month high in the dollar fuelled export concerns for U.S. crops, analysts said.

U.S. grains and soybeans face stiff competition for export business from ample global supplies, and domestic ending stocks are expected to be large when the U.S. Department of Agriculture issues crop forecasts at an annual conference that begins on Thursday, analysts said.

The USDA is separately slated to issue weekly export sales data on Thursday.

"The dollar index has risen dramatically, causing U.S.-origin grains and oilseeds to look expensive," said Andrew Whitelaw at agricultural consultants Episode 3.

The most-active soybean contract was down 13-1/2 cents at $11.72-3/4 a bushel by 11:40 a.m. CST (1740 GMT). It earlier reached $11.71-1/2, the weakest price since December 2020 and below a previous three-year low set last week.

CBOT corn was down 8-1/2 cents at $4.22-1/4 a bushel, the lowest price for a most-active contract since December 2020.

Speculative funds hold large short positions, or bets that prices will fall, in the markets, which are already down about 10% each this year.

"The funds haven't been smacked for being short," said Matt Wiegand, commodity broker for FuturesOne. "They have plenty of incentive to add to their position."

CBOT wheat was also weaker, with the most-active contract falling 16-3/4 cents to $5.80-3/4 a bushel after earlier sliding to its lowest in nearly a month at $5.77-1/2.

"European prices are staying at the low. The dollar is sharply higher," Wiegand said.

"The fund selling is able to push it lower. Until they stop getting rewarded, they're going to keep piling in."

Euronext wheat eased to fresh contract lows as export competition from the Black Sea region hung over a European market burdened with high stocks. (Reporting by Tom Polansek in Chicago. Additional reporting by Gus Trompiz in Paris and Peter Hobson in Canberra; Editing by Sherry Jacob-Phillips, Kirsten Donovan and Jan Harvey)