Randomly listing entries in more then one section I have created 12 sponsorship ads for my website using a “sponsor” category.

On my home page I have four sponsorship sections, each holding three sponsorship ads.

How can I randomly distribute those 12 sponsorship ads among the four sponsorship areas with each homepage load?
I would just slice the randomized array of categories:

{% set ads = craft.categories({
    group: 'ads',
    order: 'RAND()'
}) %}

{% for ad in ads|slice(0, 2) %}
    {{ ad.externalUrl }}
{% endfor %}

{% for ad in ads|slice(2, 4) %}
    {{ ad.externalUrl }}
{% endfor %}