#back {
  width: 900px;
  height: 700px;
  background-color:#9060AF;
  position: relative;
  margin: auto;
}

#content {
  width: 800px;
  height: 600px;
  top: 0px;
  position: relative;
  margin: auto;
}

#box1 {
  position: absolute;
  top: 490px;
  left: 100px;
}

#box2 {
  position: absolute;
  top: 490px;
  left: 355px;
}

#box3 {
  position: absolute;
  top: 490px;
  left: 610px;
}