JSFiddle - React, Tailwind, and code Playground

HTML

<input id="input">
<button id="go">Go</button>
<p id="result"></p>

JavaScript

const id = (s) => document.getElementById(s);
const rand = (n) => Math.random() * (n + 1) | 0;

/***/
id("go").addEventListener("click", function() {
  let n = +id("input").value;
  
  let a = rand(n);
  let b = rand(n - a);
  let c = n - a - b;
  
  id("result").textContent = [a, b, c].join(", ");
});