SOURCE

<div class="flex">
    <div class="flex_item">
        <p> FLEX </p>
    </div>
    <div class="flex_item">
        <p> FLEX </p>
    </div>
    <div class="flex_item">
        <p> FLEX </p>
    </div>
</div>

.flex {
   display: flex;
   flex-wrap: wrap;
   background-color: #eee;
   justify-content: center;
   align-items: start;
}
.flex_item{
    width: 4rem;
    height: 1rem;
    background: #000;
    margin: 1vmin;
    color: #fff;
    position: relative;
}
.flex_item > p {
    position: absolute;
    margin: 0;
    padding: 0;
    top: calc(50% - 1rem);
    left: calc(50% - 1rem);
    line-height: 1rem;
}
div:last-child {
    background: #aaa;
}