I'm assuming your select menus look similar to this:
<select name='blah' onchange='blah'>
<option>blah</option>
<option>blah blah</option>
[other options]
</select>
Wrap ALL of the select menus in a form like this:
<form method='post' action='/'>
[all 3 select menus here]
</form>
Then make sure the onChange looks exactly like this:
onChange="checkMenu(this.options[this.selectedIndex].value);this.form.reset()"
If this doesn't work, please post an example and I'll take a look.
-Snib -
http://www.snibworks.com
Where will you be in 100 years?