JSFiddle - React, Tailwind, and code Playground

HTML

<!-- CHANGE PRICE BELOW & HIT 'RUN' TO CHECK -->

<!-- Working jsfiddle version: v35 -->
<!-- Working jsfiddle for UAT: v36 -->

<div class="price">£99.99</div>
<div class="Instal1"></div>
<br>
    <div class="Instal2"></div>
<br>
<div class="Instal3"></div>

CSS

.price {
    display:none
}

JavaScript

jQuery(document).ready(function () {

var handyTotal = jQuery("#ordersummary table td.balanceToPay div").text().replace(/\u00A3/g, '');

var aPrice = handyTotal / 3; // takes basket total & /3
var aPrice2dec = parseFloat(aPrice.toFixed(3)); // converts to number with 3 decimal places
var bPrice = handyTotal / 3;
var bPrice2dec = parseFloat(bPrice.toFixed(3));
var cPrice = handyTotal / 3;
var cPrice2dec = parseFloat(cPrice.toFixed(3));  
    
var bPriceFloored = Math.floor(bPrice2dec); // rounds down number 
var cPriceFloored = Math.floor(cPrice2dec);
var overflowbPrice = bPrice2dec - bPriceFloored; // finds amount to add to a
var overflowcPrice = cPrice2dec - cPriceFloored;
var calcaPrice = jQuery(".Instal1").text((parseFloat(aPrice2dec) + parseFloat(overflowbPrice) + parseFloat(overflowcPrice)).toFixed(2)); // adds b+c to a
    
var bPriceFlooredDiv = jQuery(".Instal2").text(Math.floor(bPrice2dec));
var cPriceFlooredDiv = jQuery(".Instal3").text(Math.floor(cPrice2dec));
    
});

//Intstal1 = calcaPrice // Instal2 = bPriceFloored // Instal3 = cPriceFloored