This is not very hard to do but it can't be done all the time. This is easiest done on the very first Classic board. There needs to be a setup on the board that allows both droppers and all 3 collectors to be involved in a path. There is no other requirement; you do not have to cause enough splits that both droppers dump droplitz into all 3 collectors at the same time. It could be as simple as feeding one dropper to one collector, the other dropper to another collector, and a U-Dial to the third. As long as all top droppers and all bottom collectors are engaged at the same time, you'll satisfy the requirement for a Full House. Besides being an achievement, this grants a huge point reward.