JSFiddle - React, Tailwind, and code Playground

HTML

<header>
<div>1</div>
<div>2</div>
<div>3</div>
<span>Right</span>
</header>

CSS

header
    {
     display: flex;
    }

    header > div
    {
     margin-right: auto;
    }

    header > span
    {
     margin-left: auto;
    }