.javaNoob, если я правильно понял.
<!doctype html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style>
.error {
display: none;
}
</style>
</head>
<body>
<form action="">
<input type="radio" class="radio" />#1<br />
<input type="radio" class="radio" />#2<br/>
<input type="radio" class="radio" />#3<br /><br />
<button type="submit">Отправить данные</button>
</form>
<div class="error">Поля не заполнены!</div>
<script>
document.forms[0].onsubmit = function() {
var radioElems = document.querySelectorAll('.radio'),
error = document.querySelector('.error');
submit = false;
[].forEach.call(radioElems, function(item) {
item.checked && (submit = true);
});
if (!submit) {
error.style.display = 'block';
return false;
}
};
</script>
</body>
</html>