SOURCE

<div class="box-wrapper">
    <div>wew</div>
<!-- <div class="box1"></div> -->
<div class="box2">kk</div>
<!-- <div class="box3"></div> -->
</div>
<style>
    .box-wrapper{
        width: 500px;
        height:150px;
        display:flex;
        flex-direction: column;
        /* align-items: center; */
        justify-content: center;
    }
    .box1{
        width: 150px;
        height:100px;
        background: red;
    }
    .box2{
        flex: 1;
        /* height: 0; */
        /* height:150px; */
        background: green;
    }
    .inn{
        /* height:100%; */
    }
    .box3{
        width: 150px;
        height:150px;
        background: blue;
    }
</style>