Ну вот набросал что-то...
<html>
<body>
<input type="checkbox" name="checkbox" id="cb1"><label for="cb1"> 1</font></label><br>
<input type="checkbox" name="checkbox" id="cb2"><label for="cb2"> 2</label><br>
<input type="checkbox" name="checkbox" id="cb3"><label for="cb3"> 3</label><br>
<input type="checkbox" name="checkbox" id="cb4"><label for="cb4"> 4</label><br>
<input type="checkbox" name="checkbox" id="cb5"><label for="cb5"> 5</label><br>
<script type="text/javascript">
var cbs = [
document.getElementById('cb1'),
document.getElementById('cb2'),
document.getElementById('cb3'),
document.getElementById('cb4'),
document.getElementById('cb5')
];
for(var i = 0; i < cbs.length; i++ ) {
cbs[ i ].onclick = function() {
var count_checked = 0;
for(var i = 0; i < cbs.length; i++ ) {
if ( cbs[ i ].checked ) {
count_checked++;
}
}
for(var i = 0; i < cbs.length; i++ ) {
if ( count_checked >= 3 ) {
if ( !cbs[ i ].checked ) cbs[ i ].disabled = true;
} else {
cbs[ i ].disabled = false;
}
}
}
}
</script>
</body>
</html>