<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.left-box{
height:300px;
width:100px
}
.middle-box{
height:300px;
flex:1
}
.right-box{
height:300px;
width:150px
}
</style>
</head>
<body>
<div style='width:600px;height:600px;border:solid 1px black;display: flex;'>
<div class='left-box'>
</div>
<div class='middle-box'>
</div>
<div class='right-box'>
</div>
</div>
</body>
</html>