<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title></title>
<style>
ul{
display:flex;
}
li:first-child,
li:last-child{
50px;
background-color: green;
}
li:nth-child(2){
flex:1;
background-color: yellow;
}
</style>
</head>
<body>
<ul>
<li>1</li>
<li>2</li>
<li>3</li>
</ul>
</body>
</html>