It depends a bit on the current way of storing these files.
Did you store each image from a video in a corresponding folder?
If so, how would you like to sample the files, if their amount is not divisible without a remainder by 3? Would you like to drop the last files or fill it up with repetition?
Should the frames be contiguous in a sample or would you like to shuffle the frames from a single video?
Also, would you like to shuffle the video folders or should the videos be loaded in a consecutive way?