A: Hello
A. 3 codes is Gold plan.
B. When you buy codes on AppSumo, go to your profile page and open "Products" -> "Neuronwriter". You can have as many products as codes and in each product will be a code.
C. Click on the REDEEM button next to code(s) or open page: https://app.neuronwriter.com/redeem-code
and activate your codes.

Hope this helps ;)