<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<div class="content">
<input type="text" class="data">
<input type="text" class="names">
<br></div>
<br>
<button id="add">Добавить</button>
<button id="send">Отправить</button>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script>
$('#add').on('click', function(){
$('.content').append('<input type="text" class="data"><input type="text" class="names"><br>');
});
$('#send').on('click', function(){
if([].some.call($('input'), el=> el.value.length < 3)) alert("Не все поля заполнены");
});
</script></body>
</html>