<style>
    body {
        font-family: 'MinervaSans';
    }

    .container {
        display: flex;
        border: 1px solid salmon;
        width: 500px;
    }

    .box {
        width: 100px;
        height: 100px;
        border: 1px solid black;
    }
</style>
<div class="container">
    <div class="box" style="flex-grow: 4;">1 I grow the most</div>
    <div class="box" style="flex-grow: 2;">2 I grow</div>
    <div class="box" style="flex-grow: 0;">3 I don't</div>
</div>