Search wordpress using three different drop down menus

You should be using different variables in your URL query, such as:

mysite.com/?year=2001&make=Chevrolet&model=Express

Make sure the name=”” in your select inputs reflect the name changes. You will then just need to read in the variables using PHP’s $_GET or $_REQUEST global variable. Something like this

$year = ($_GET['year']) ? $_GET['year'] : false;
$make = ($_GET['make']) ? $_GET['make'] : false;
$model = ($_GET['model']) ? $_GET['model'] : false;

You can then use the $year, $make, $model variables in your query if they are not false.

Make sure you also do the proper escaping and filtering of the $_GET variables as well, for security reasons.