By Tim Kelly TOKYO (Reuters) – Japan, Britain and Italy will sign a treaty in Tokyo next week to establish a joint organisation and industry group for developing their planned advanced jet fighter, three sources with direct knowledge of the plans said. An inter-government body will oversee the industry group venture led by Britain’s BAE Systems PLC, Japan’s Mitsubishi Heavy Industries and Italy’s Leonardo. That group will distribute work to teams in different parts of the Global Combat Air Programme (GCAP), such as the engine and avionics, the sources said. They asked not to be identified beca…