SOURCE

<div class="container">
    <div class="item">1</div>
    <div class="item">2</div>
    <div class="item">3</div>
</div>

.container { /* 父 */
    display: flex;
    align-items: flex-start; /*flex-start | flex-end | center | baseline | stretch*/
    width: 400px;
    height: 400px;
    background-color: pink;
}
.item { /* 子 */
    width: 100px;
    height: 100px;
    border: 1px solid #000;
    background-color:red;
}