JSFiddle - React, Tailwind, and code Playground

HTML

<div id="left-side">Left blah blah balh</div>
<div id="content">Fluid centre</div>
<div id="right-side">Right yep</div>

CSS

#left-side {
    float:left;
    width:auto;
    max-width:300px;
    background:red;
}
#content {
    float:left;
    background:blue;
}
#right-side {
    float:left;
    width:auto;
    max-width:300px;
    background:pink;
}

JavaScript

var content = document.getElementById('content'),
    left = document.getElementById('left-side').offsetWidth + 20,
    right = document.getElementById('right-side').offsetWidth + 20,
    total = left + right;

function resize() {
content.style.width = (window.innerWidth-total) + "px";
}
resize();
window.onresize = resize;