Another gallery question I have looked around but cant find an answer to this.

I have a structure for my gallery page, in-which there are three separate entries called A, B and C. 

There are different images within these entries. But how would I go about showing all the images within these entries in a random order? 

Say I went to the gallery page and they all showed up randomly, then theres a sub navigation to the separate entries on there own?

It's probably a simple solution but templating it has me really confused. 
Thank you
You can set an array and order them by random.

{% set gallery = craft.entries.section(['galleryA','galleryB','galleryC']) %}

Then you have to display them by random using .order('RAND()')

So your for loop looks like:

{% for entry in craft.entries.section(gallery).order('RAND()') %}