Today, we are done with a form where selection of an option in first drop-down enabled the second which on changed enabled the third drop-down. "onchange()" function was incorporated in the template to achieve this. Earlier, all drop-downs had all the database entries corresponding to category table. Well now first one shows only the database fields that are required i.e. labs.
For the imminent implementation for the filters in the other two drop-downs, we will be using AJAX.