.container {
  width:100%;
  overflow:hidden;
}

#slider {
  left:0;
  transition:left 500ms ease-in;
  width:300%;
  padding-top:35%;
  position:relative;
}

.slide {
  position:absolute;
  top:0;
  width:33.33%;
  height:100%;
  background-size:cover;
}

#slide1 {
  left:0%;
  background-color:red;
}

#slide2 {
  left:33.33%;
  background-color:green;
}

#slide3 {
  left:66.66%;
  background-color:blue;
}