Okay, first you'll need three random generators that draw 12 items per interview, each. Use them in the questionnaire and you know which videos to display.
In the easy case, create 3 times 12 pages for the videos plus the questions do display after each set. We could as well work with
loopPage() to do it with one page per condition, but it's really hard to get that working together with setPageorder() for the condition order.
So, if your random generators are names RG01, RG02 and RG03 and the video filenames are stored in the generators (as items), the video-pages will contain a PHP code like this:
replace('%videofile%', 'RG01x1', 'response');
And on the second page in condition 1
replace('%videofile%', 'RG01x2', 'response');
Up to the the 12th video in the third condition
replace('%videofile%', 'RG03x12', 'response');
Below that PHP code, you'll need the HTML code to display the video (HTML 5), and just replace the filename by the placeholder %videofile%.
When that works, give the first an last page in each set (12+x) a page ID and before the first page, use the PHP code as described here: Rotation of Question Blocks.
Finally, if you need the order of the three conditions, read on here: put () Funktion um Rotation festzuhalten (you'll need either code (1) or (2).