PHOENIX—A fiery three-vehicle crash in which six people were killed and five others injured occurred after one driver who had a flat tire told her friend driving a second vehicle to change directions by crossing a freeway median to pull over and get help, authorities said. The friend and her five children were killed in the July 2 wreck on Interstate 10 west of Phoenix, while the woman with the flat tire was seriously injured along with her four children in her vehicle, the Arizona Department of Public Safety said in a probable-cause statement released Friday. Though there was ample paved space on the sides of the freeway to stop, the two vehicles both crossed the median in a no U-turn zone and then were going about 15 mph in a traffic lane when they were struck from behind by a big rig under the speed limit at 73 mph, …