<!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(){
var a = 0;
$('input').each(function(){
if($(this).val().length<3) a = 1;
});
if(a) alert("Не все поля заполнены");
});
</script>
</body>
</html>