TODAY The Democratic National Committee holds three days of meetings, beginning today, at the Shoreham Hotel. The American Health Planning Association holds a three-day conference at the Hyatt Regency Hotel, beginning today at 10:30 a.m. NASA and the Council of Chief State School Officers hold a 10:45 a.m. briefing at the L'Enfant Plaza Hotel on NASA's Teacher in Space Project. The President's Commission on Organized Crime holds three days of hearings in New York City on the involvement of organized crime in gambling. The hearings will be held at the U.S. Court of International Trade, Appellate Courtroom, 1 Federal Plaza, beginning at 9:30 a.m. The Environmental Policy Institute holds a 10 a.m. briefing on the potential transportation risks of a proposed nuclear-waste storage facility in Tennessee. The news conference will be held at its offices at 218 D St. SE. The Cato Institute sponsors a forum on "The Urgency of International Tax Relief," at 4 p.m. at 224 Second St. SE. Phone 546-0200. The National Bureau of Standards sponsors a two-day seminar on improving access to Japanese technical information. Phone 921-3181. The National Oceanic and Atmospheric Administration, the National Institutes of Health and the National Fisheries Institute sponsor a three-day conference on the cardiovascular benefits of fish oil, beginning at 9 a.m. today at the Capitol Holiday Inn, 550 C St. SW. TUESDAY The leaders of the nation's 10 oldest and largest environmental groups unveil their "Environmental Agenda for the Future" at an 11 a.m. news conference at the Wilderness Society, 1400 I St. NW. Former CIA director Stansfield Turner speaks on the problems of conducting intelligence activities in an open society at an 8 a.m. breakfast briefing sponsored by the Sheraton Grand Hotel and Capital Speakers Inc. The session will be held in the Sagamore Room of the hotel. Phone 393-0772. THURSDAY The Republican National Committee's three-day summer meeting starts at the Westin Peachtree Plaza Hotel in Atlanta. The National Consumers League and the George Mason University Public Choice Center sponsor a forum to review the report of the National Academy of Sciences on the carcinogenicity of cyclamates. The session begins at 9:30 a.m. at 1530 P St. NW.