Hi Kenneth,
Unfortunately this functionality is not supported by the RadDock control and there no easy, out of the box, solution.
The RadDock and the RadDockZone are rendered as a <div> HTML elements on the page. Basically we have <div> (RadDock) within a <div> (RadDockZone). If you can make the <div>s behave as you have described there would not be a problem making the dock's behave the same way.
I think that, you might be able to do it using JavaScript, but you need to know when any of the docks are resized so that you can recalculate the height of the bottom-most (the third in your case) dock.
Best wishes,
Pero
the Telerik team