GRAINS-Soybeans at three-week low as U.S. harvest weighs

* Soybean prices hit lowest since Sept. 13 on higher yields

* Additional pressure on soybean prices from improved Brazil weather

* Wheat falls for fourth session, corn extends losses

(Updates with European trading, changes byline/dateline) PARIS/SINGAPORE, Oct 3 (Reuters) - Chicago soybean futures dropped for a second session on Tuesday, falling to their lowest in almost three weeks amid reports of strong yields from an advancing U.S. harvest. U.S. corn also edged lower in the face of supply pressure from harvesting, although slower-than-expected field work last week and rain forecast for the coming days lent some support. Wheat edged higher, consolidating after a pullback fueled by a bigger-than-expected spring wheat crop estimate last Friday from the U.S. Department of Agriculture (USDA). The most-active soybean contract on the Chicago Board of Trade had fallen 0.2 percent to $9.55-3/4 a bushel by 1218 GMT, after dropping earlier in the session to its lowest since Sept. 13 at $9.52-1/2. CBOT corn declined 0.2 percent to $3.50-3/4 a bushel while wheat gained 0.3 percent to $4.46-1/4. "First yield reports are showing for the moment better potential than expected," consultancy Agritel said regarding the corn and soybean harvest. "Some analysts are revising up their yield perspectives, approaching the USDA estimates which appeared too optimistic until now." Brokerage INTL FCStone raised its estimate of the U.S. 2017 corn yield to 169.2 bushels per acre (bpa), from 166.9 bpa in its previous monthly report released on Aug. 31. The firm also raised its forecast of the U.S. 2017 soybean yield to 49.9 bpa, from 49.8 bpa previously. In a weekly crop report issued after the market close on Monday, the USDA said the U.S. soybean harvest was 22 percent complete by Sunday, behind an average of trade estimates for 25 percent. The corn harvest was 17 percent complete, also trailing an average analyst estimate of 21 percent. The focus is turning to South America, where planting for the 2018 soybean crop is underway, and reports of beneficial rain in Brazil were also curbing soy prices. "The rain means most Brazilian soybean regions will have enough rain to support rapid planting. Brazil's soybean crops now look more likely to avoid most of the problems that would arise with late planting," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia. In Argentina, a dry spell for the next 10 days should bring some relief to waterlogged fields of soybeans and corn after rains over the weekend added to fears of planting delays, experts said on Monday. The wheat market faced pressure from higher estimates of the U.S. spring wheat crop. But tenders being held on Tuesday by importing countries Egypt and Tunisia suggested healthy international demand, even if Black Sea origins where expected to dominate the tenders.

Prices at 1218 GMT

Last Change Pct End Ytd Pct Move 2016 Move CBOT wheat 446.25 1.50 0.34 408.00 9.38 CBOT corn 350.75 -0.75 -0.21 352.00 -0.36 CBOT soy 955.75 -1.50 -0.16 1004.00 -4.81 Paris wheat Dec 166.75 1.25 0.76 175.00 -4.71 Paris maize Nov 155.75 0.75 0.48 170.00 -8.38 Paris rape Nov 367.00 0.25 0.07 383.25 -4.24 WTI crude oil 50.44 -0.14 -0.28 53.72 -6.11 Euro/dlr 1.17 0.00 0.16

Most active contracts - Wheat, corn and soy US cents/bushel, Paris futures in euros per tonne

(Reporting by Naveen Thukral in Singapore and Gus Trompiz in Paris; Editing by Dale Hudson)