<style>
/* 代码编写区域 */

</style>
<div class="main">
    <div>1</div>
    <div>2</div>
    <div>3</div>
</div>
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先动手实现下，在查看参考答案哦。
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body {
    margin: 0;
}
.main {
    width: 100vw;
    height: 100vh;
    background: skyblue;
    display: flex;
    align-items: center;
}
.main div:nth-of-type(1) {
    width: 100px;
    height: 100px;
    background: pink;
}
.main div:nth-of-type(2) {
    flex-grow: 1;
    background: greenyellow;
    align-self: stretch;
}
.main div:nth-of-type(3) {
    width: 100px;
    height: 100px;
    background: pink;
}
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