KABUL, Afghanistan — Three Taliban militants were killed on Friday when a roadside bomb they were planting exploded prematurely, as Afghan and foreign troops launched an operation to retake a neighboring Afghan town controlled by Taliban militants, officials said. The blast occurred in Kandahar, south of Kandahar city on a road frequently used by NATO and police forces, he said. In Musa Qala, ground forces surrounded the area and airstrikes targeted militants inside the town in the southern province of Helmand, defense ministry spokesman Gen. Mohammad Zahir Azimi said. "The operation has started," said Maj. Charles Anthony, a spokesman for...