indendi qualcosa del genere:
codice:
<html>

<head>
<script>
function modifica(questo){
var x=questo.selectedIndex;

if (questo.name=='D1'){

switch(x){
  case 0:
	document.modulo.D2.selectedIndex=1;
	document.modulo.D3.selectedIndex=1;
	break;

  case 1:
	document.modulo.D2.selectedIndex=2;
	document.modulo.D3.selectedIndex=2;
	break;

  case 2:
	document.modulo.D2.selectedIndex=0;
	document.modulo.D3.selectedIndex=0;
	break;
  }
 }

if (questo.name=='D2'){

switch(x){
  case 0:
	document.modulo.D3.selectedIndex=1;
	break;

  case 1:
	document.modulo.D3.selectedIndex=2;
	break;

  case 2:
	document.modulo.D3.selectedIndex=0;
	break;
  }
 }
}
</script>

</head>

<body>

<form name="modulo">
<select size="1" name="D1" onChange="modifica(this)">
<option>uno</option>
<option>due</option>
<option>tre</option>
</select>



<select size="1" name="D2" onChange="modifica(this)">
<option>uno</option>
<option>due</option>
<option>tre</option>
</select>



<select size="1" name="D3">
<option>uno</option>
<option>due</option>
<option>tre</option>
</select>
</form>

</body>

</html>
?