api.php

<?php

hm_add_rewrite_rule( array(
    'request_method'    => 'DELETE',
    'regex'             => '/api/v3/posts/([^/]+)',
    'query'             => 'post_id=$matches[1]',
    'request_callback' => function( WP $wp ) {

        wp_delete_post( $wp->query_vars['post_id'] );

        exit;
    }
) );