Flexbox simple

HTML

<div class="flex-container">
    <div class="a">A</div>
    <div class="b">B</div>
    <div class="c">C</div>
</div>

CSS

.flex-container {
    background-color: #3333CC;
    display: -webkit-flex;
    display: flex;
}

.flex-container > * {
    background-color: #33CC33;
    margin: 10px;
}

.a,
.c {
    flex-grow: 1;
}

.b {
    flex-grow: 2;
}