WUERZBURG, Germany—Three people were killed and five seriously injured in a knife attack in southern Germany on Friday by a 24-year-old Somali immigrant, described by officials as having been placed under compulsory psychiatric treatment in recent days. Police stopped and arrested the attacker in the town of Wuerzburg by firing a bullet through his thigh. They said his injuries were not life-threatening. “Three are dead and five very seriously injured,” regional Interior Minister Joachim Herrmann said after arriving on the scene. “With the most seriously injured we are not sure they will survive.” The man had lived in Wuerzburg since 2015, the year Germany opened its borders to more than a million migrants and refugees fleeing war and poverty. His life was not in danger and he was being questioned by police in hospital, Herrmann said. “His condition had been noticed in recent months, including violent tendencies, and a few …