one of the coolest things about jquery is its ability to handle events in the DOM.
the event you're looking for is .change() http://api.jquery.com/change/
the html markup for 2 select elements
<select id="first_select">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<select id="second_select" disabled="disabled">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
javascript/jquery to handle changes to the select boxes
$("#first_select").change(function() {
$("#second_select").removeAttr('disabled');
});
The second select element will be enabled when a change is made to the first. Follow this logic for the third select.