Edit in JSFiddle

<div class="flex-container">
<div class="flex-item item1">flex: 1</div>
<div class="flex-item item2">flex: 1</div>
<div class="flex-item item3">flex: 1</div>
</div>

.flex-container {
    display: -webkit-flex;
    display: flex;
    width: 300px;
    height: 240px;
    background-color: Silver;
}

.flex-item {
    background-color: DeepSkyBlue;
    margin: 5px;
    -webkit-flex: 1;
    flex: 1;
}