参考链接：
https://www.cnblogs.com/qingchunshiguang/p/8011103.html
练习代码
<!DOCTYPE html>
<html lang="en" dir="ltr">
<head>
<meta charset="utf-8">
<title></title>
<style media="screen">
.container{
background-color: orange;
300px;
height:300px;
display:flex;
justify-content:space-between;
align-items: center;
}
.div1{
background-color: red;
100px;
height:100px;
}
.div2{
background-color: blue;
100px;
height:100px;
}
.div3{
background-color: green;
100px;
height:100px;
}
</style>
</head>
<body>
<div class="container">
<div class="div1">
</div>
<div class="div2">
</div>
<div class="div3">
</div>
</div>
</body>
</html>