In NGUI 3, the draw order is always based on depth by default. All widgets are first sorted by panel depth, then by widget depth.
If widget depths are the same, what will be on top of what will not be certain.
If the panel depth is the same, it's the same thing -- draw order will not be clear.
All this is assuming that your panels are set to "Automatic" Render Q mode. If it's something else, then the draw order may be based on transform's Z position (distance to the camera), or a combination of the two methods. Leave it on Automatic for best results.
