.rellax2 {
  background-image:url("foto2.jpg");
  background-size: cover;
  height: 100vh;
  width: 100%;
  z-index: 1;
  top: 100vh;
  position: absolute;
}
.rellax1 {
  background-color: green;
  height: 100vh;
  width: 100%;
  position: absolute;
  z-index: 2;
}
.rellax3 {
  position: absolute;
  top: 200vh;
  height: 100vh;
  background-color: green;
  width: 100%;
  z-index: 2;
}

body {
  background-color: yellow;
  background-attachment: fixed;
}