<?php
$ancestors_array = get_ancestors($category_id, 'category');
$very_first_ancestor_id = end($ancestors_array); // Cat3
$parent_id = reset($ancestors_array); // Cat3.1
<?php
$ancestors_array = get_ancestors($category_id, 'category');
$very_first_ancestor_id = end($ancestors_array); // Cat3
$parent_id = reset($ancestors_array); // Cat3.1