Select attiva select che attiva select

[br1]

<script>
function abilita(obj) { 
	if (obj.name=='uno') {
		if (obj.selectedIndex>0) {
			obj.form.due.removeAttribute("disabled"); 
		} else {
			obj.form.due.setAttribute('disabled','true');
			obj.form.tre.setAttribute('disabled','true');
		}
	}
	if (obj.name=='due') {
		if (obj.selectedIndex>0) {
			obj.form.tre.removeAttribute("disabled"); 
		} else {
			obj.form.tre.setAttribute('disabled','true');
		}
	}
}
</script>
<form>
<select name="uno" onchange="abilita(this)"> 
<option value=""> - scegli - </option>
<option value="1">uno.1</option>
<option value="2">uno.2</option>
</select>


<select name="due" onchange="abilita(this)" disabled> 
<option value=""> - scegli - </option>
<option value="1">due.1</option>
<option value="2">due.2</option>
</select>


<select name="tre" disabled> 
<option value=""> - scegli - </option>
<option value="1">tre.1</option>
<option value="2">tre.2</option>
</select>
</form>

ciao