Three U.S. wrestlers started competition at the Junior World Freestyle Wrestling Championships in Istanbul, Turkey, August 29. On the first day, the United States team won all four of its matches.    The Junior World Championships feature the world's best wrestlers who are 17-20 years old.    Winning two matches on Friday was Don Reynolds (New Lenox, Ill./Overtime WC) at 74 kg/163 lbs. Reynolds beat Sanjar Shodmanov of Uzbekistan, 8-0, then stopped Tyler Marghetis of Canada, 6-1.    According to coach Troy Steiner, Reynolds' next match should be in the quarterfinals, most probably against an athlete from Russia.    Two other wrestlers won their first matches of the tournament, Paul Donahoe (Davison, Mich.) at 50 kg/110 lbs. and Nate Gallick (Tucson, Ariz./Cyclone WC) at 60 kg/132 lbs.    Donahoe scored a 10-0 technical fall over Frank Osei Mensah of Canada. His next opponent is scheduled to be Evgeni Nikanorov of Israel.    Gallick opened up with a 8-2 decision over Luis Ortiz of Puerto Rico. His next opponent will be Bevilacqua Cianmarco of Italy.    No other U.S. wrestlers competed on Thursday.    The U.S. athletes are being coached by Steiner, who lives in Madison, Wis., along with Donny McPherson of Portland, Ore.    U.S. Junior World Freestyle Team results    50 kg/110 lbs. - Paul Donahoe (Davison, Mich.) - tech. Fall over Frank Osei Mensah (Canada), 10-0    60 kg/132 lbs. - Nate Gallick (Tucson, Ariz./Cyclone WC) - dec. Luis Ortiz (Puerto Rico), 8-2    74 kg/163 lbs. - Don Reynolds (New Lenox, Ill./Overtime WC) - dec. Sanjar Shodmanov (Uzbekistan), 8-0; dec. Tyler Marghetis (Canada), 6-1    U.S. athletes who have not yet started competition  55 kg/121 lbs. - Mike Silengo (Colorado Springs, Colo./Dave Schultz WC)  66 kg/145.5 lbs. - Andy Simmons (Williamston, Mich./Michigan WC)  84 kg/185 lbs. - Kurt Backes (Neshanic Station, N.J./Cyclone WC)  96 kg/211.5 lbs. - Nathan Moore (Indianapolis, Ind./Purdue)  120 kg/264.5 lbs. - Cole Konrad (New London, Wis./Minnesota Storm)  Coaches: Troy Steiner (Madison, Wis.), Don McPherson (Portland, Ore.)