Edit in JSFiddle

#container {
    position: relative;
    width: 200px;
    height: 200px;
    border: solid 1px black;
}

#box1 {
    float: left;
    width: 50%;
    height: 150px;
    background: red;
}

#box2 {
    float: left;
    width: 40%;
    height: 100px;
    background: blue;
}

#box3 {
    float: left;
    width: 30%;
    height: 70px;
    background: yellow;
}

<div id="container">
    <div id="box1"></div>
    <div id="box2"></div>
    <div id="box3"></div>
</div>