CHICAGO, May 20 (Reuters) - Chicago wheat fell for a third straight session on Friday, retreating further from a two-month high hit earlier this week as technical selling pressured the market, traders said.

Corn also eased, as accelerated U.S. planting and news that Argentina may expand an export volume cap weighed.

Soybeans gained on strong export demand, amid tight supplies.

The most-active wheat contract on the Chicago Board of Trade (CBOT) lost 31-3/4 cents at $11.68-3/4 a bushel, a 0.7% weekly decline.

CBOT corn ended 4-1/2 cents lower at $7.78-3/4 a bushel, ending lower for a third consecutive week.

Soybeans firmed 14-3/4 cents to $17.05-1/4 a bushel, logging a weekly gain of 58-3/4 cents, a 3.6% increase.

Global wheat supplies continue to face weather challenges. In the United States, an annual field tour of Kansas this week found the lowest yield potential in the top winter wheat state since 2018.

"Traders became very nervous up there, particularly the speculative funds," said Arlan Suderman, chief commodities economist at StoneX. "U.S. exports are basically priced out of the market."

Global export constraints could be easing, as India is considering allowing traders to ship out wheat sitting at ports after a sudden ban on exports prevented dealers from loading cargoes.

The United Nations is attempting to broker a deal to allow Ukrainian grain to ship from Black Sea ports that have been closed since Russia's invasion in February which, along with a bumper Russian harvest, could also ease a potential shortfall in wheat supply.

Soybeans remain supported by strong export demand amid tightening U.S. old crop stocks.

"When you look around the world at other oilseeds as a replacement, beans are still relatively cheap," said Mark Schultz, chief analyst at Northstar Commodity. "There still is pretty good demand."

Oilseed markets were digesting the latest policy announcement by top palm oil producer Indonesia, which said it would reinstate a requirement to allocate a certain volume for the domestic market as it lifts a recent export embargo. (Reporting by Christopher Walljasper Editing by Marguerita Choy)