No visa is required by USA citizens for entry into Namibia and Botswana. You will need only your passport. For Zimbabwe you will need to purchase a tourist visa upon entry into the country either at the airport or at the border post. The visa cost is USD 30.00
At the Immigration desk when entering the countries, you will be required to hand in a completed Arrival form. Most of the questions are straight forward, but they will want to know where you will be staying while in the country. Use the following address:
For Namibia:
Khaya Guesthouse, 507 Voortrekker Road, Okahandja, Namibia
For Botswana:
Chobe Safari, Kasane, Botswana
For Zimbabwe:
The Kingdom Resort, Victoria Falls, Zimbabwe