With two pivots, you can start the partitioning at either end and have a well defined location for each of the three partitions (at the top, at the botton, and inbetween). With more than three partitions, you would not know where to put the second to second last border without first sorting the array. You could just guess, but that would probably result in quite a lot of additional array shifting (lucky if you have a dedicated blitter chip for that :-)).