I cannot imagine what kind of input resolution woiuld be need in order to use radius 3, if it doesn't exist such device maybe would be better to not let the slider go so high.
Low frequency images (soft edged or images with little edge detail) benefit–particularly in combination with an edge mask. I often use a higher radius (1.5-2.5) when sharpening certain portraits and using an edge mask between 60-70.
As for why it goes so high...for the same reason Amount goes to 150 and Brightness goes to 150. Sometimes you need more...