.box-1 {
    Width: 600px;
    height: 400px;
    
    } 
.first{
    background-color: blue;
    width: 200px;
    height: 100%;
    float: left;
}
.second{
    background-color: yellow;
    width: 200px;
    height: 100%;
    float: left;
}
.third{
    background-color: red;
    width: 200px;
    height: 100%;
    float: left;
}