Ok, so you will need three HTML <select> tags and an <input type='submit'> tag placed inside a <form> tag in your HTML page. In your PHP, you can use the $_POST array to retrieve posted data like so:
if( isset( $_POST[ "first_select" ]))
$first_selection = $_POST[ "first_select" ];
// assuming the name of your first <select> tag is "first_select"
$first_select = false;
// or some other default value
You will probably need to validate the data that has been posted, regular expressions are often useful for this purpose.
Have a go and if you have any more questions please don't hesitate to ask. :)