siwals what about http://nbh.localhost/forum/api/discussions/3 ?
or http://nbh.localhost/forum/public/api/discussions/3 if you didn't use a virtualhost/alias with its root in the public folder, which you should do in production 😉
EDIT: didn't notice you used flagrow/flarum-api-client. I don't think you need the /api suffix when creating the client. Also it should work with beta 8 without any change, but I don't think many people used it with beta 8 yet.