Sum 3 Numbers- HTML

<!DOCTYPE html>
<html>
<head>

 <script src="test1.js"></script>

</head>

<body>

<br>First Number: <input type="number" name="firstnum" id=1></br>
<br>Second Number: <input type="number" name="secondnum" id=2></br>
<br>Third Number: <input type="number" name="thirdnum" id=3></br>
<button type="button" onclick="sumNumbers()">Try it</button>
</body>
</html>