Three people were killed and one was critically injured after a helicopter carrying National Guard members and a Border Patrol agent crashed in southern Texas on Friday while following people who were crossing the border with Mexico, officials said.The helicopter, which crashed at 2:50 p.m., was conducting an operation near Rio Grande City, according to Joint Task Force North, an operation under the Defense Department that supports Customs and Border Protection with National Guard units.Two of the dead were soldiers, and the third was a Border Patrol agent, Joint Task Force North said in a…