un ejemplito rápido
Código HTML:
<html>
<head>
<script>
function setDisplay(opcion){
if (opcion==0){
document.forms[0].input1.style.display="block";
document.forms[0].input2.style.display="none";
document.forms[0].input3.style.display="none";
}
else if (opcion==1){
document.forms[0].input1.style.display="block";
document.forms[0].input2.style.display="block";
document.forms[0].input3.style.display="none";
}
else if (opcion==2){
document.forms[0].input1.style.display="block";
document.forms[0].input2.style.display="block";
document.forms[0].input3.style.display="block";
}
}
</script>
</head>
<body>
<form>
<select onchange="setDisplay(this.selectedIndex)">
<option>primero</option>
<option>segundo</option>
<option>tercero</option>
</select>
<input type="text" style="display:block" name="input1"><br>
<input type="text" style="display:none" name="input2"><br>
<input type="text" style="display:none" name="input3"><br>
</form>
</body>
</html>