sequence of stimuli

I am trying to build an experiment with images. In every step the participants will see a sequence 0f 3 stimuli (650 ms for a stimulus with 250 ms isi) and afterward they will have 1000 ms gap and then they will see another sequence of 3 stimuli. In the end of the step they will have to choose the first sequence or the second one. I can’t find a way to present in every step 2 different sequence.
I will really appreciate a help
thanks, neta

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