Sonya,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<input type="number" step="3" id="summa" min="3" value="5">
<p>Итого: <span id="count"></span></p>
<script>
window.addEventListener("DOMContentLoaded", function() {
var item = document.getElementById("summa");
function foo() {
var output = document.getElementById("count");
output.innerHTML = (this.value / 3).toFixed(2);
}
item.addEventListener("input", foo);
foo.bind(item)();
});
</script>
</body>
</html>