erilar,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<output></output>
<form action="http://" name="form">
<input class="class_input" type="text" value="Инпут 1">
<input class="class_input" type="text" value="Инпут 2">
<input class="class_input" type="text" value="Инпут 3">
</form>
<script>
const showValue = _ => {
let inputs = document.querySelectorAll('.class_input');
let valInput = [...inputs].map(({value}) => value);
document.querySelector('output').value = valInput
}
document.form.addEventListener("input", showValue);
showValue();
</script>
</body>
</html>