<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
* {
box-sizing:border-box;
}
.left {
background-color:blue;
padding:10px;
float:left;
width:25%;
}
.center {
background-color:lightgray;
padding:10px;
float:left;
width:50%;
}
.right {
background-color:red;
padding:10px;
float:left;
width:25%;
}
@media screen and (max-width:800px) {
.left, .center, .right {
width:100%;
}
}
</style>
</head>
<body>
<div class="left">
<p>Left</p>
</div>
<div class="center">
<p>Center</p>
</div>
<div class="right">
<p>Right</p>
</div>
</body>
</html>