Edit in JSFiddle

<div class="container">
    <div class="B B1">B1</div>
    <div class="B B2">B2</div>
    <div class="B B3">B3</div>
</div>
.container {
    margin:10px;
    display: -webkit-flex;
    display: flex;
    width: 500px;
    height: 240px;
    background-color: #eee;
}
.B {
    width: 100px;
    height: 100px;
}
.B1{
    background-color:rgba(255,255,0,.5);
    -webkit-flex-grow:1;
    -moz-flex-grow:1;
    flex-grow:1;
}
.B2{
    background-color:rgba(255,0,255,.5);
}
.B3{
    background-color:rgba(0,255,255,.5);
}