ENCOURAGED by the ease of holding up a jewellery shop and escaping with the loot in broad daylight, a gang of robbers returned to the shop less then a week later and raided it again, the High Court has heard. One of the gang, Sin Muk-kan, 30, was sentenced to a total of 15 years' jail yesterday for his role in three robberies. He had denied three counts each of robbery and possession of an imitation firearm at the time of the holdups but was found guilty after trial on Thursday by a jury. Sentencing Sin, Mr Justice Leong said the robberies, which involved nearly $1.4 million worth of gold ornaments, were well planned. Although only imitation guns were used, he said police might think they were genuine and open fire, putting by-standers at risk. Senior Crown Counsel Tim Casewell said the first raid occurred at Yue Hing Jewellery Company in Western on May 26, 1991, and the other two took place at the Fordy Jewellery Company, also in Western, on August 14 and 20, 1991. On all occasions, masked men burst into the shops, threatened the occupants with ''pistols'', which were later found to be toy guns, snatched the gold ornaments from the showcases and escaped in cars.