I’m surprised, I thought that was how it was supposed to work if the ‘loopType’ was random (i.e. rows 0:3 would be the first of a random selection of rows).
If you’re sure that it doesn’t work, then an alternative would be to set up a random loop with no selected rows, but terminate the loop when three trials have been completed. To do this, insert a code component on your Presentation routine, and put something like this in the End routine tab:
if trials.thisN == 2: # i.e. trials 0, 1, 2 have been completed
trials.finished = True # end the loop early