body>
<select id="first">
<option value="">선택</option>
<option value="a">a</option>
<option value="b">b</option>
</select>
<select id="second" disabled>
<option value="">선택</option>
<option value="1">1</option>
<option value="2">2</option>
</select>
<select id="third" disabled>
<option value="">선택</option>
<option value="ㄱ">ㄱ</option>
<option value="ㄴ">ㄴ</option>
</select>
</body>
<script>
$("#first").change(function(){
if(this.value!=""){
$("#second").removeAttr("disabled");
}else{
$("#second").attr("disabled", "disabled");
$("#third").attr("disabled", "disabled");
}
});
$("#second").change(function(){
if(this.value!=""){
$("#third").removeAttr("disabled");
}else{
$("#third").attr("disabled", "disabled");
}
});
</script>