What do you mean with three months amount? Certainly you don't have to book a camp site for a period of three months.

But I guess you mean three months in advance. This depends on what kind of campsite. For privately owned campsites it is often not necessary to book in advance. Unless in the middle of a Namibian holiday. If there are no other options in the area it might be better to book in advance. Otherwise just go to the next campsite.
For the government campsites, you often have to book in advance. How long in advance depends on time of year and location. During Namibian summer holidays places like Mile 72 and mile 108 can be fully booked. At other times of year, you might be the only one there.
In Etosha you have no alternatives to the government sites. And you have to book in advance. Specially for the period july-october.
There is very little local transport and distances are huge. Namibia is not a typical backpacker country. Best thing is to hire your own car. For most of the country you do not need a 4x4 and both cars renatls and fuel is inexpensive.
Greetings,