I couldn't say without looking at your database. I can see I get results when I choose Mobile, HP...
You probably want to add the same onclick event to the first select list as the second, so changing either refreshes the third list, but that isn't the problem with no results.
If you add this above your select list on your plugin page it will write the query to the page so you can test it in phpMyAdmin and that might help you figure out what is wrong with it:
<?php echo($Model_list->debugSQL()); ?>