div {
    width: 1200px;
    height: 900px;
    position: relative;
    margin-top: 0px;
    float: left;
   
}

#block1 {
    background: black ;
    width: 400px ;
    position: relative;
    margin-left: 0px;
    margin-bottom: 0px;


}
#block2 {
    background: gold ;
    width: 400px ;
    position: relative ;
    margin-left: 0px ;

}
#block3 {
    background: red;
    width: 400px ;
    position: relative;
    margin-left: 0px ;

}