THE Trade Development Council, Hong Kong's official trade promoter, is to triple the number of offices on the mainland to 15 within the next two to three years to lend more support to local companies tapping trade and investment opportunities. The TDC will add an office each in Dalian and Wuhan this year to the existing ones in Beijing, Shanghai, Guangzhou, Shenzhen and Tianjin. Later it plans to set up in Qingdao in Shandong province, Fuzhou or Xiamen in Fujian province, and either Chengdu or Chongqing in Sichuan province. The other cities have yet to be chosen. Mary Wong, the TDC's assistant director in charge of China operations, said the offices would be set up as soon as ''we can find the people and the space''. Except for Tianjin, which will be upgraded later from a consultation office, the other four existing outfits are full offices. The expansion is central to the TDC's overall strategy for developing Hong Kong as a bridge between China and the rest of the world as European and American investors are increasingly enamoured of the Chinese market. And because many are unfamiliar with Chinese practices, they are turning more to Hong Kong-based companies and expertise to set up joint ventures to monitor that market. Details of the TDC's China strategy will be unveiled in a two-day ''China Trade Open Day'' scheduled for January 18 and 19 at the Convention Plaza, with China's Vice-Minister for Internal Trade He Jihai as the guest of honour. At the seminar, the TDC will both identify the business opportunities in various sectors for Hong Kong businesses, and help to match the firms with mainland companies. In the past year, it organised more than 70 events to help the territory's companies expand their businesses on the mainland. These included trade missions, trade shows in major department stores, seminars, fashion shows and exhibitions. The strategy to expand the TDC network throughout China comes as Hong Kong-based manufacturers and traders are extending beyond southern China, where the territory's investments are largely based. Increasingly, the territory's investors are stepping up their presence by going as far as the Yangtze and provinces such as Sichuan and Heilongjiang. The Yangtze region - which follows China's longest waterway and is one of the country's most populous areas - is at the centre of an ambitious plan to copy Guangdong's success. The push for the plan will come chiefly from Shanghai, where the central authorities are concentrating their financial reforms to help the city regain its pre-1949 status as the premier financial centre. Hong Kong's retailers and property developers have been quick to take their cue and have set up department stores and launched big property projects in the city. Among the Hong Kong companies which have set up operations there are Sincere, Top Form and Ryoden. Retailers said that despite the high cost of doing business in Shanghai, they could not afford to miss out as the city was a fashion leader where buyers from all over the country congregated. Tianjin, Dalian, Qingdao, Wuhan, Nanjing and Harbin are also becoming favourites with Hong Kong retailers, thanks partly to the TDC, which has taken an active role in introducing firms to the Chinese market since 1989. Local companies such as Goldlion, Crocodile, Giordano, Yaohan, Pacific Concord, Watson's, Wellcome and Park'N Shop have set up there to tap Chinese consumers' increasing purchasing power.