Force drag and drop order - is it possible?

Hello... still playing with the trial version of storyline.

Here is what I would like to do:

Have three columns of drag and drop items.

I would like items in the first column to be dragged first. THEN items from the second column, THEN items from the third column.

I am not quite sure how to achieve this. Should I use triggers, layers, "hiding"? I am still all at sea with those three concepts... - I would like to control things with variables but it is all a bit mysterious still. I wish there were a straightforward scripting language, but I guess what I am looking for is doable somehow.

Thanks for any help

Andrew