THE emerging trend of promoting a Hongkong-South China-Macau tourism triangle is a welcome development for hotels in Guangzhou. Traditionally, Guangzhou hotels have always lived through a boom-slump-boom cycle, with the Canton Trade Fair filling up the rooms twice a year, allowing the hotels to coast through the remaining months with far more modest occupancy levels. However, with the fair now scaled down to a 10-day event from 15 days, and with easy access to China's export centres no longer making a grand showcase so important to businessmen, the city's hotel industry is suffering a drop in revenue. Although occupancy levels are still high during the spring and autumn event, and some hotels are still able to raise their rates by 200 per cent, the revenue is no longer so dependable. The first move in what could become a major tourism triangle policy was announced in April by the Hongkong Tourist Association (HKTA), when it stated that it had decided to promote Hongkong together with China. The HKTA's move will mean that Hongkong will primarily join forces with the Guangdong region, working with tour operators and hotels there. Although it will, initially, be Hongkong that benefits, in the longer term the advantages to the Pearl River delta are going to be considerable. At present about 37 per cent of tourists to Hongkong make a side trip to China and, with the right kind of promotion, that figure could be considerably increased. ''The potential for Hongkong, Macau, and the whole of South China really is enormous,'' said Mr Rory Scott, director of InterTour '93 which is due to open on June 3 in the Hongkong Convention and Exhibition Centre. ''It widens the whole scope of what Hongkong has to offer and, with the territory being the gateway to Southern China, what could be a better or a more logical move?''