Hi @andreasb,
Thanks for the post.
You will only need to add the third field to the $GLOBALS[] class like so:
Passing the third field’s key and name like so:
// array of filters (field key => field name)
$GLOBALS['my_query_filters'] = array(
'field_1' => 'color',
'field_2' => 'size',
'field_3' => 'weight'
);