I'm not sure what you are trying to do with this...
$query = "SELECT brandname FROM products, product_subcategory, product_gencategory
WHERE (((product_gencategory.gencategory)='Stationery') AND ((product_subcategory.subcategory)='Disk'))";
$result = mysql_query($query) or die("query failed");
That's the way I'd write it
Mark