JSFiddle - React, Tailwind, and code Playground

HTML

<select name="Test1">
    <option value=1>1</option>
    <option value=5>5</option>
    <option value=10>10</option>
</select>

<select name="Test2">
    <option value=1>1</option>
    <option value=5>5</option>
    <option value=10>10</option>
</select>

<select name="Test3">
    <option value=1>1</option>
    <option value=5>5</option>
    <option value=10>10</option>
</select>

<br/>

<input type="text" value="0" name="TotalSum">

JavaScript

$("select").change(function() {
    var combined = 0;
 $("select[name='Test1'],select[name='Test2'],select[name='Test3']").each(function() {
        combined += parseInt(this.value);
    });
    $("input[name='TotalSum']").val(combined);
}).trigger("change");