<!DOCTYPE html>
<html>
<title>Заголовок документа</title>
<head>
<style>
.box {
width: 350px;
height: 200px;
padding-left: 0;
list-style-type: none;
border: 1px dashed black;
display: -webkit-flex;
display: flex;
}
.box div {
flex: 1;
}
.green {background-color: #8ebf42}
.blue {background-color: #1c87c9;}
.gray {background-color: #666}
</style>
</head>
<body>
<h2>Пример свойства flex</h2>
<div class="box">
<div class="green">GREEN</div>
<div class="blue">BLUE</div>
<div class="gray">GRAY</div>
</div>
</body>
</html>