* CBOT corn on course for 8% weekly gain after sharp rebound * U.S. reported this week 3.7 mln T in corn export sales to China * Adverse S. America crop weather also supports corn, soybeans (Updates with European trading, adds byline/dateline) By Gus Trompiz and Enrico Dela Cruz PARIS, Jan 29 (Reuters) - Chicago corn futures rose on Friday to hold near the previous session"s 7-1/2 year high as massive Chinese purchases and doubts over South American harvest prospects sustained buying interest. Soybeans bounced after dropping on Thursday, similarly underpinned by Chinese demand and South American crop risks. Wheat was also firm. Traders said all three crops remained prone to erratic movements as resistance levels after multi-year highs and worries about volatility in equity markets were countering support from tightening grain supply. The Chicago Board of Trade (CBOT) most-active corn contract was up 1.5% at $5.42-3/4 a bushel by 1125 GMT. The benchmark contract, which rose to $5.50-1/2 on Wednesday, was on course for a weekly gain of more than 8% after rebounding from a sell-off last Friday. The U.S. Department of Agriculture this week confirmed three separate sales of U.S. corn to China, totalling 3.74 million tonnes, or 147 million bushels, for delivery in the 2020-21 year that ends on Aug. 31. The huge sales have put the focus back on booming Chinese imports and tightening U.S. supplies ahead of an uncertain South American harvest season. "We need to ration demand and with the correction in prices (last week) we saw the opposite," Benjamin Bodart, analyst with Agritel, said. "We still see some upside in corn and soybeans for the moment. Wheat is more a follower." Hot weather prompted the Buenos Aires Grains Exchange to shave its estimate for Argentina"s soy crop to 46 million tonnes, from 46.5 million tonnes earlier. In Brazil, soybean harvesting delays would likely continue throughout February after a drought pushed back plantings, Hedgepoint Global Markets said. The market is also concerned about rain in parts of Brazil hampering harvest work. CBOT soybeans rose 0.9% to $13.66 a bushel and wheat added 0.8% to $6.52 a bushel. The wheat market was waiting to see the impact of an upcoming Russian export tax and monitoring rumours that Argentina could curb exports to safeguard domestic supply. Prices at 1125 GMT Last Change Pct End Ytd Pct Move 2020 Move CBOT wheat 652.00 5.00 0.77 640.50 1.80 CBOT corn 542.75 8.25 1.54 484.00 12.14 CBOT soy 1366.00 12.75 0.94 1311.00 4.20 Paris wheat Mar 229.00 -0.75 -0.33 213.25 7.39 Paris maize Mar 216.00 0.25 0.12 198.50 8.82 Paris rape Feb 455.00 1.75 0.39 418.25 8.79 WTI crude oil 52.44 0.10 0.19 48.52 8.08 Euro/dlr 1.21 0.00 0.06 1.2100 0.23 Most active contracts - Wheat, corn and soy US cents/bushel, Paris futures in euros per tonne (Reporting by Gus Trompiz in Paris and Enrico Dela Cruz in Manila; Editing by Rashmi Aich, Kirsten Donovan)