MixPetrov,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<link rel="stylesheet" href="https://ajax.googleapis.com/ajax/libs/jqueryui/1.11.4/themes/smoothness/jquery-ui.css">
<script src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.11.4/jquery-ui.min.js"></script>
<script>
$ (function () {
function mathC(){
var nmb = +this.dataset.nmb;
var ns = +this.value;
ns = ns || 0;
ns = Math.ceil(ns/nmb)*nmb;
this.value = ns
}
var timer;
$('.quantity_cart [type="text"]').on("input", function() {
window.clearTimeout(timer);
timer = window.setTimeout(mathC.bind(this), 1200)
})
});
</script>
</head>
<body class="quantity_cart">
<input name="1" size="2" value="1" type="text" data-nmb="3">
<input name="1" size="2" value="1" type="text" data-nmb="3">
</body>
</html>