<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<div class="content">
<input type="text">
<input type="text">
<input type="text">
<input type="text">
<br>
</div>
<br>
<button id="send" disabled>Отправить</button>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script>
$('input').on('input', function(){
var a = 0;
$('input').each(function() {
if(this.value.length<3) {
a = 1;
this.style.backgroundColor = 'red';
}
else this.style.backgroundColor = 'green';
});
$('#send').prop('disabled', a);
});
$('#send').on('click', function(){
$("body").empty();
$("body").append("<h2><center>Спасибо!</center></h2>");
});
</script>
</body>
</html>