* Strong demand, tightening export supplies underpin wheat

* Grain market eyes next week's USDA supply-demand report

CHICAGO, Nov 5 (Reuters) - Chicago soybean futures fell for a third day on Friday, pressured by harvest activity and strong planting progress in South America that may open up export competition by mid-January, analysts said.

Corn also eased from harvest pressure, while wheat stepped back after reaching a nine-year high earlier in the week.

Corn and soybeans were capped by expectations that the USDA will increase its U.S. harvest forecasts.

The Chicago Board of Trade's (CBOT) most active soybean contract fell 12-1/4 cents to $11.97 a bushel as of 11:41 a.m. CDT (1641 GMT), poised for a 3% weekly decline.

CBOT corn was down 4 cents at $5.55-1/4 a bushel, while wheat lost 5-1/2 cents to $7.68-1/4 a bushel.

For the week, corn looked to fall 2.4%, while wheat was poised for a slight decline.

After rain delays pushed back corn and soybean harvests, this week has offered a clear window for field work, said John Zanker, market analyst at Risk Management Commodities.

"We should get most of the beans out by the middle of next week, when it starts to rain again," he said. "We’re starting to see some lines at the elevators, that we weren’t seeing earlier, as we have run out of room on the farm."

South American planting progress also pressured soybeans, as Brazil looks to finish seeding ahead of schedule, pushing up potential harvest and shrinking the U.S. export window.

"If their season goes according to plan, they’re going to have new crop available in mid-late January. Then we’re done," said Tom Fritz, commodity broker with EFG Group.

Corn also suffered from harvest pressure, but remained underpinned by rising fertilizer prices that could affect farmers' planting decisions next spring.

"It’s not a stretch to think we’re going to cut corn acres," said Zanker. "With input costs rising sharply, and competition from not only soybeans, but cotton, oats and sorghum, it’s going to be tough to get the acreage we need.

Wheat remained near nine-year highs, supported by global supply concerns, as dry growing conditions in Russia and Ukraine threaten recently sown winter crops after poor spring harvests this year.

Global wheat demand continues to be strong with higher-than-expected purchases by top buyers, though some traders see a possible correction as expected crops in Argentina and Australia bolster supply, while late corn harvests in the Northern Hemisphere meet livestock feed needs.

Rain in Argentina over the past week helped developing 2021/2022 wheat crops, the Buenos Aires Grains Exchange said on Thursday.

However, there was concern heavy rain could damage Australia's wheat crop, bringing the risk of knocking over maturing wheat crops in the eastern third of the country's wheat belt, Commodity Weather Group said in a note. (Reporting by Christopher Walljasper; additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; editing by Jonathan Oatis)