Millisecond Forums

I'm trying to program a choice task- I have three categories of images, that I've paired with each other in three blocks (Ha, Hb, Ub). Is there a way to randomize the presentation of all of these images… e.g., an equal likelihood that a pairing from any of these blocks will be presented next, instead of randomized sequentially by three separate blocks? I am not sure how to organize the trials to do this as they're currently set up… would I have to move all of the pairs to a single block to do that? Part of the current script is attached here.