JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html" 
<body>
<form action="" name="numbers">
    <table>
        <tr>
            <td>Number One</td>
            <td>
                <input type="text" name="numOne" value="0" size="5" onchange="calcAvg();" />
            </td>
        </tr>
        <tr>
            <td>Number Two</td>
            <td>
                <input type="text" name="numTwo" value="0" size="5" onchange="calcAvg();" />
            </td>
        </tr>
        <tr>
            <td>Number Three</td>
            <td>
                <input type="text" name="numThree" value="0" size="5" onchange="calcAvg();" />
            </td>
        </tr>
        <tr>
            <td>Average of Three Numbers</td>
            <td>
                <input type="text" name="averageResult" value="0" size="5" />
            </td>
        </tr>
    </table>
    </body>
    <script>
        function calcAvg() {
            var numberOne = document.numbers.numOne.value;
            var numberTwo = document.numbers.numTwo.value;
            var numberThree = document.numbers.numThree.value;
            var calcResult = 0;
            calcResult = (parseInt(numberOne) + parseInt(numberTwo) + parseInt(numberThree)) / 3;
            document.numbers.averageResult.value = calcResult;
        }
    </script>