JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="vol1" size="4";>
<input type="text" id="vol2" size="4";>
<input type="text" id="vol3" size="4";>
<input type="button" onclick="unit1()" value="Saberi">

<span id="rezultat"></span>

JavaScript

function unit1(){
var x = +document.getElementById("vol1").value,
    y = +document.getElementById("vol2").value,
    z = +document.getElementById("vol3").value;
    var rez= x+y+z;
    document.getElementById("rezultat").innerHTML=(""+ rez )
}