BEIJING has stopped three leading intellectuals from going to the United States to participate in a conference on Sino-Hong Kong relations. Tong Dalin, Wu Mingyu and Zheng Zhongbing were scheduled to have taken part in a two-day seminar, ''Hong Kong and Greater China'', which will open in Princeton University on October 23. The symposium will be hosted by the renowned academic grouping, Future China Society, in conjunction with departments and workshops in the university. Chinese sources said yesterday the authorities had prevented the trio from going because of the ''political sensitivity'' of the conference, whose participants included liberal politicians from Hong Kong as well as overseas-based dissidents. Both Mr Tong and Mr Wu, respectively the head of the Research Society for the Reform of the Economic Structure, and the former vice-chief of the Economic Development Centre of the State Council, are veteran economists with ministerial ranks. A former editor with the now-defunct journal New Observer, Zheng is a liberal author. ''Tong and Wu have in the past couple of years travelled widely to the West, including the US,'' an informed source said. ''However, they have been advised by the authorities to avoid the conference. ''Zheng's unit, the Chinese Writers' Association, has privately voiced opposition to his going.'' A major theme of the conference, which will be attended by Hong Kong legislators Szeto Wah, Emily Lau Wai-hing and Christine Loh Kung-wai, will be ''the problem of 1997'' and post-1997 Sino-Hong Kong relations. Other participants include dissident intellectuals already in the US such as Liu Binyan, Wang Ruoshui, Li Honglin, Sun Changjiang and Chai Ling. The former head of Hong Kong's Xinhua (New China News Agency) Xu Jiatun, will also be at Princeton during the time, even though he has declined to take part in the conference as a formal participant. Director of the Future China Society and an organiser of the conference, Ruan Ming, said in Princeton last night he regretted the fact that his three good friends could not join the symposium. ''Some people in Beijing may feel that it is too sensitive for the three to take part in the seminar,'' said Mr Ruan, a one-time secretary to the late party chief Hu Yaobang. ''However, the event is entirely academic in nature, and has no relationship with the pro-democracy movement.'' Mr Ruan said the conference was open to the public, and he welcomed diplomats from Chinese missions in the US to take part. Sources in Washington said several Congressmen and their aides would attend the conference, which would have an impact on the US Government's assessment of whether Hong Kong would enjoy a high degree of autonomy after 1997. China analysts said the ''ban'' on the three intellectuals was in a pattern with a series of actions taken by Beijing to contain dissent and to thwart closer ties between intellectuals in China and their counterparts in Hong Kong and overseas.