I have 2 groups of blocks: a group "A" with 12 blocks from which I randomly choose 3 blocks to present to a participant and other group "B" with 6 blocks from which I also randomly choose 3 blocks to present to a participant. The problem was that when I randomized these 2 groups the platform was presenting to a participant 3 group A blocks together and 3 group B blocks together, although it changed the group order between participants, but I want that the 6 blocks presented to a participant be mixed between groups A and B (for instance: A1, B3, B4, A9, etc.) 
With the help of ahmedA, I created a branched survey with embedded counters that almost works, mixing blocks from each group. The problem now is that whenever the counter of A (or reaches the limit of 3, if the randomizer chooses another A (or , the survey ends even if it didn't reach 6 blocks in total, that is, even if B (or A) didn´t reach the limit of 3, instead of moving to another group. I'm sending you a simplified QSF (based on a QSF courtesy of ahmedA) and I would very much appreciate some help on this. 
MTADSLB_v2.qsf