For a terse, pure flexbox option, group the left-aligned items and the right-aligned items:
<div class="wrap">
<div>
<span>One</span>
<span>Two</span>
</div>
<div>Three</div>
</div>
and use space-between:
.wrap {
display: flex;
background: #ccc;
justify-content: space-between;
}
This way you can group multiple items to the right(or just one).