They don't have to have a target listed in form view, but if they aren't listed, they can't be dragged. If you can't drag your other two objects, there is little point in having them.
Objects on the screen cannot be dragged, unless they have a trigger attached to them (such as "When object is dragged over", or "When object is dropped on") or they are listed in the form view as a drag item.
you said:
There probably is an obvious answer to this question, but I don't know it, and can't find it!
Experiment and see what happens; you may find something you like better than the "real"answer. :)