List with categories, subcategories and posts of custom posttype

@niko Please, find your required code, it will give you exact output you were seeking for. I think you were doing all right except in the final output you were missing the right structure.

Also, I recommand not to assign posts to the parent category as this structure not support that. However, I think with minor tweaks in following that can also be acheived.

<?php   
    $taxonomy = 'testimonial-category';
    $postType="testimonial";
    $terms = get_terms(['taxonomy' => $taxonomy, 'orderby' => 'term_id', 'parent' => 0, 'hide_empty' => false]);
?>
<div class="">
<?php
    foreach($terms as $term){
        echo '<h3>' . $term->name . '</h3>';
        $childTerms = get_terms(['taxonomy' => $taxonomy, 'orderby' => 'term_id', 'parent' => $term->term_id, 'hide_empty' => false]);

        foreach($childTerms as $childTerm)
        {
            $posts = get_posts(array('post_status' =>'publish','post_type' => $postType,
                    array(
                        'taxonomy' => $taxonomy,
                        'field' => 'term_id',
                        'terms' => $childTerm->term_id,
                    ),));
            ?>
            <div class="add-accordion">
                <h3><?php echo $childTerm->name ?></h3>
                <div class="add-accordion">
                    <?php foreach($posts as $post){ ?>
                        <h3><?php echo $post->post_title ?></h3>
                        <div class="">
                            <?php echo get_the_content($post->ID) ?>
                        </div>
                    <?php } ?>
                </div>
            </div>
            <?php
        }
    }
?>
</div>
<script>
    jQuery(function(){
        jQuery('.add-accordion').accordion({
            collapsible: true,
            heightStyle: "content",
            active: false,
            animate: 500,
            icons: false
        });
    });
</script>
<?php wp_enqueue_script('jquery-ui-accordion'); ?>