SACRAMENTO, Calif. — Eleven people were wounded and one killed after a concert in Dallas. Five more people were hurt when they were shot on a rural road in South Carolina. Sunday’s horrific melee in Sacramento was just one of three mass shootings in the United States over the weekend — evidence, some experts believe, that extreme cases of gun violence are becoming so routine that they almost fade into the background. “We only hear about some of these shootings,” said James Densley, a criminal justice professor in Minnesota who runs a mass-shooting database called The Violence Project. “It kind…