The US military has shot down a third flying object over North American airspace in three days, as the air force general overseeing the airspace said he would not rule out any explanation for the objects yet. The high-altitude unidentified object, described as an “octagonal structure” with strings attached to it, was shot down over Lake Huron in Michigan on Sunday. It is understood to have been the same item that was picked up by radar over Montana on Saturday. At the point it was struck by an air-to-air missile launched by F-16 fighter jets, it had been flying across the Great Lakes region at 20,000ft, a height that could…