Impossible to invoke a method on array. How to merge multiple objects and keep as objects
You can achieve this with Craft's shuffle() method and Twig's slice filter (like you mentioned in your answer).

{% set newsletterImages = newsletter.imagesCongratulations.all() %}
{% set newsletterImages = newsletterImages | merge(newsletter.imagesCis.all()) %}
{% set newsletterImages = newsletterImages | merge(newsletter.imagesSection3.all()) %}

{% set shuffledImages = shuffle(newsletterImages) %}  // Shuffle our "master" array

{% for image in shuffledImages[:6] %}  // Get the first 6 images
  {{ image.title }}&lt;br&gt;
{% endfor %}