场景和需求

屏幕 > 540px，前两个部分展示两列，
屏幕 < 540px，前两个部分展示一列
屏幕 < 540px，第一部分要反转展示
屏幕 > 769px，第三个部分展示三列
屏幕 < 769px，第三个部分展示一列

效果图

请添加图片描述

实现方式一：改变 flex-direction（推荐）

<!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>
     * {
      margin: 0;
      padding: 0;
     }

     body {
      display: flex;
      flex-direction: column;
      gap: 1em;
      padding: 1em;
     }

    .container {
      display: flex;
      gap: 1em;
    }

    .container > * {
      flex: 1;
    }

    .container > :first-child {
      background-color: cyan;
    }
    

    @media (max-width: 540px) {
    
      .container--col-2-reverse {
        flex-direction: column-reverse;
      }
      
      .container--col-2 {
        flex-direction: column;
      }
    } 

    
    @media (max-width: 769px) {
      .container--col-3 {
        flex-direction: column;
      }
    } 



  </style>
</head>
<body>
  <div class="container container--col-2-reverse">
    <div>
      Happy birthday!!!party1
    </div>
    <div>
      Happy birthday!!!party2
    </div>
  </div>
  <div class="container container--col-2">
    <p>
      Lorem ipsum dolor sit amet consectetur adipisicing elit. Dignissimos ratione eligendi repellendus illo temporibus corrupti, error ullam ipsam magni necessitatibus quas animi dolore placeat, voluptatibus, reiciendis obcaecati rem unde quos!
    </p>
    <p>
      Lorem ipsum dolor sit amet consectetur adipisicing elit. Dignissimos ratione eligendi repellendus illo temporibus corrupti, error ullam ipsam magni necessitatibus quas animi dolore placeat, voluptatibus, reiciendis obcaecati rem unde quos!
    </p>
  </div>
  <div class="container container--col-3">
    <p>
      Lorem ipsum dolor sit amet consectetur adipisicing elit. Dignissimos ratione eligendi repellendus illo temporibus corrupti, error ullam ipsam magni necessitatibus!
    </p>
    <p>
      Lorem ipsum dolor sit amet consectetur adipisicing elit. Dignissimos ratione eligendi repellendus illo temporibus corrupti, error ullam ipsam magni necessitatibus quas animi dolore placeat, voluptatibus, reiciendis obcaecati rem unde quos!
    </p>
    <p>
      Lorem ipsum dolor sit amet consectetur adipisicing elit. Dignissimos ratione eligendi repellendus illo temporibus corrupti, error ullam ipsam magni necessitatibus quas animi dolore placeat, voluptatibus, reiciendis obcaecati rem unde quos!
    </p>
  </div>
  
</body>
</html>

实现方式二：改变 flex-wrap + flex-basis（一般）

<!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>
     * {
      margin: 0;
      padding: 0;
     }

     body {
      display: flex;
      flex-direction: column;
      gap: 1em;
      padding: 1em;
     }

    .container {
      display: flex;
      gap: 1em;
    }

    .container > * {
      flex: 1;
    }

    .container > :first-child {
      background-color: cyan;
    }
    

    @media (max-width: 540px) {

      
      .container--col-2-reverse {
        flex-wrap: wrap-reverse;
      }

      .container--col-2-reverse > * {
        flex-basis: 100%;
      }

      .container--col-2 {
        flex-wrap: wrap;
      }
      
      .container--col-2 > * {
        flex-basis: 100%;
      };

    } 

    
    @media (max-width: 769px) {
      .container--col-3 {
        flex-wrap: wrap;
      }

      .container--col-3 > * {
        flex-basis: 100%;
      }
    } 



  </style>
</head>
<body>
  <div class="container container--col-2-reverse">
    <div>
      Happy birthday!!!party1
    </div>
    <div>
      Happy birthday!!!party2
    </div>
  </div>
  <div class="container container--col-2">
    <p>
      Lorem ipsum dolor sit amet consectetur adipisicing elit. Dignissimos ratione eligendi repellendus illo temporibus corrupti, error ullam ipsam magni necessitatibus quas animi dolore placeat, voluptatibus, reiciendis obcaecati rem unde quos!
    </p>
    <p>
      Lorem ipsum dolor sit amet consectetur adipisicing elit. Dignissimos ratione eligendi repellendus illo temporibus corrupti, error ullam ipsam magni necessitatibus quas animi dolore placeat, voluptatibus, reiciendis obcaecati rem unde quos!
    </p>
  </div>
  <div class="container container--col-3">
    <p>
      Lorem ipsum dolor sit amet consectetur adipisicing elit. Dignissimos ratione eligendi repellendus illo temporibus corrupti, error ullam ipsam magni necessitatibus!
    </p>
    <p>
      Lorem ipsum dolor sit amet consectetur adipisicing elit. Dignissimos ratione eligendi repellendus illo temporibus corrupti, error ullam ipsam magni necessitatibus quas animi dolore placeat, voluptatibus, reiciendis obcaecati rem unde quos!
    </p>
    <p>
      Lorem ipsum dolor sit amet consectetur adipisicing elit. Dignissimos ratione eligendi repellendus illo temporibus corrupti, error ullam ipsam magni necessitatibus quas animi dolore placeat, voluptatibus, reiciendis obcaecati rem unde quos!
    </p>
  </div>
  
</body>
</html>