* CBOT wheat contracts find contract lows

* March corn falls 2%

* Soybeans touch three-week low

(New throughout, updates prices, market activity and comments to U.S. trading; new byline, changes dateline, previous HAMBURG)

CHICAGO, Nov 27 (Reuters) -

Chicago grain and soybean futures tumbled on Monday after the Thanksgiving holiday weekend, pressured by slow demand and technical selling and as traders liquidated positions ahead of Thursday's first notice day for deliveries.

Chicago Board of Trade March corn futures and all wheat contracts fell to contract lows while soybeans declined to a three-week low before rising slightly.

"There's liquidation," said Dan Basse, president of AgResource Co in Chicago. "We pushed new lows in wheat and corn, sparking new selling and that is pretty much the story."

Chicago Board of Trade wheat fell 2.7% to $5.61-3/4 a bushel. Corn declined 1.7% to $4.55-1/2 a bushel after sinking to $4.53-3/4, the lowest for a most-active contract since December 2020. Soybeans were up 0.2% at $13.33-1/2 a bushel at 1741 GMT after earlier sliding to their lowest since Nov. 2 at $13.23-1/2 a bushel.

"We're kind of stuck on a holiday affair here," said Dale Durchholz, a private commodity risk consultant in Bloomington, Illinois. "The volume and interest in trading has started to go down at this point so things can get a little bit sloppy simply because there is not a lot of volume and liquidity to offset it."

Soybeans found some support out of Brazil, where 2023/24 planting had reached 74% of the expected area as of Thursday, agribusiness consultancy AgRural said on Monday, making it the

slowest progress

for the period in eight years as the country grapples with bad weather.

The market continues to closely monitor weather in the top exporter nation, where hot, crop-threatening conditions remain in the forecast.

Some new purchasing demand for wheat was seen, with international tenders issued on Monday by Pakistan and Bangladesh. But cheap Black Sea wheat, especially from Russia, was tipped to win the business. (Additional reporting by Michael Hogan in Hamburg, Naveen Thukral; Editing by David Gregorio)