Car registration with desired identification mark

The question certainly fits in here net, but I did not find a more suitable forum….. The question in itself is whether it is right, or on which law it is based, the particularly “short” mark only for motorcycles and US-cars to be awarded, or which law I can invoke to get a certain “free” mark. Background of the thing is that I now do not return my old mark (vehicle was j In order to explain the background, that’s why I’m so sick of it….. -I have 3 Sierras with the following license plates **-EE 3, **-EE 4, **-EE 6 -if I have to start now with a “Z” license plate (for Zwickau-Land), I want to change it all immediately and then continuously, however, it should also bring something for me, I want to have one place less than before, a Is the example of the Z-EE 1, Z-EE 2 and Z-EE 3, only came the statement that I could get a 3-digit net. Therefore, what is the legal basis for me to get these marks net???