<style>

​

.grid {

    display: grid;

    grid-template-columns: repeat(3, minmax(max-content, 1fr));

    grid-template-rows: repeat(3, 20px);

    border-top: 5px solid red;

    border-left: 5px solid red;

    width: 100%;

}

.grid__item {

    border-right: 5px solid red;

    border-bottom: 5px solid red;

}

</style>

​

<div class="grid">

    <div class="grid__item"></div>

    <div class="grid__item"></div>

    <div class="grid__item"></div>

    <div class="grid__item"></div>

    <div class="grid__item"></div>

    <div class="grid__item"></div>

    <div class="grid__item"></div>

</div>