JSFiddle - React, Tailwind, and code Playground

HTML

<div class="top">top</div>
<div class="left">left</div>
<div class="right">right</div>

CSS

.top {
    height: 3em;
    position: fixed;
    background: green;
    left:0;
    right:0;
}
.left {
    position: absolute;
    left: 0;
    top: 3em;
    bottom: 0;
    width: 300px;
    background: blue;
}
.right {
    position: absolute;
    left: 300px;
    top: 3em;
    bottom: 0;
    right: 0;
}