hook_preprocess_html.php

<?php
/**
 * Implements hook_preprocess_html().
 */
function hook_preprocess_html(&$vars) {
    $vars['theme_hook_suggestion'] = 'html__custompage';
}

hook_preprocess_node.php

<?php
/**
 * Implements hook_preprocess_node().
 */
function module_name_preprocess_node(&$vars) {
    // Use different template
    $vars['theme_hook_suggestions'][] = "node__type_content__custom";
}

hook_preprocess_page.php

<?php
/**
 * Implements hook_preprocess_page().
 */
function hook_preprocess_page(&$vars) {
    $vars['theme_hook_suggestion'] = 'page__custompage';
}