Paginetta di prova:

<HEAD>
<script>
function ctrl() {
f = document.mioForm;
m = f.milan;
r = f.roma;
l = f.lazio;
m.disabled = false;
r.disabled = false;
l.disabled = false;
if (m.checked && r.checked) l.disabled=true;
if (l.checked && r.checked) m.disabled=true;
if (m.checked && l.checked) r.disabled=true;
}
</script>
</HEAD>
<BODY>
<form name=mioForm>
<input type=checkbox onclick="ctrl()" name=milan>Milan

<input type=checkbox onclick="ctrl()" name=roma>Roma

<input type=checkbox onclick="ctrl()" name=lazio>Lazio

</form>
</BODY>

ciao