.container{
    position: relative;
}

.block1{
    background-color: red;
    position: absolute;
    width: 100px;
    height: 100px;
    top: 20px;
    left: 20px;
}

.block2{
    background-color: blue;
    position: absolute;
    width: 100px;
    height: 100px;
    top: 70px;
    left: 70px;
}

.block3{
    background-color: green;
    position: absolute;
    width: 100px;
    height: 100px;
    top: 120px;
    left: 120px;
    z-index: -1;
}