.carousel-inner .item:nth-child(1) {
  background: url(../img/bg_slider/bg1.jpg) no-repeat center center / cover fixed;
}
.carousel-inner .item:nth-child(2) {
  background: url(../img/bg_slider/bg2.jpg) no-repeat center center / cover fixed;
}
.carousel-inner .item:nth-child(3) {
  background: url(../img/bg_slider/bg3.jpg) no-repeat center center / cover fixed;
}