Hola liberfg.
Prueba esto:
Código PHP:
<html>
<head>
<script language='javascript'>
function accion(){
with (document.frm){
switch (ac.selectedIndex){
case 1:
s1.disabled = true;
break;
case 2:
s2.disabled = true;
break;
case 3:
s1.disabled = true;
s2.disabled = true;
break;
case 4:
s1.disabled = false;
s2.disabled = false;
break;
}
}
}
</script>
</head>
<body>
<form name='frm'>
Primero:
<select name='s1'>
<option>1</option>
<option>2</option>
<option>3</option>
</select><br>
Segundo:
<select name='s2'>
<option>A</option>
<option>B</option>
<option>C</option>
</select><br><br>
Acción:
<select name='ac' onchange='accion()'>
<option> </option>
<option>Desactiva el primero</option>
<option>Desactiva el segundo</option>
<option>Desactiva ambos</option>
<option>Reactiva ambos</option>
</select>
</form>
</body>
</html>
Saludos.