codice:
<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;

//riga che riabilita
if (m.checked ^ l.checked ^ r.checked) {r.disabled=false;m.disabled=false;l.disabled=false}

} 
</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>