Google AdSense allows maximum 3 link units. However, I have noticed that very frequently, if you add the maximum 3 link units, one of them is blank as you can see from the blank space in the right sidebar of the screen shot of this blog.
You can see the blank space at the top of the right sidebar where a Google AdSense link unit is supposed to be. It is highlighted by a red ellipse
Exactly what is going on? I will try to find out.