css/css-item-layout/1.html

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8" />
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <title>item 宽度一定，所有元素都在一个容器</title>
  <meta name="viewport" content="width=device-width, initial-scale=1">

  <style>
    .container {
      width: 430px;
      padding: 10px;
      margin: 150px auto;
      border: 2px solid rgb(240, 103, 103);
    }

    .item {
      display: inline-block;
      width: 100px; /* 宽度一定 */
      margin-right: 30px;
      margin-bottom: 20px;
      padding: 5px 10px;
      font-size: 20px;
      border-radius: 5px / 5px;
      background: rgb(103, 164, 243);
    }

    .item:nth-child(3n) {
      margin-right: 0;
    }

    .item:nth-last-child(-n+3) {
      margin-bottom: 0;
    }
  </style>

</head>
<body>
  <div class='container'>
    <div class='item'>最多放三个</div>
    <div class='item'>最多放三个</div>
    <div class='item'>最多放三个</div>
    <div class='item'>最多放三个</div>
    <div class='item'>最多放三个</div>
    <div class='item'>最多放三个</div>
    <div class='item'>最多放三个</div>
  </div>
</body>
</html>