I try to create an ad-hoc network between many Android devices, but I can\'t find documentation to do that.
I thought Wifi direct could work for what I want, but is
Android does not have support for ad-hoc WiFi at this time -- sorry!
Update (2019): Android has supported WiFi Direct for a while, and this answer has not been updated.
Your scenario would work. A would be able to connect with C as long as B (the group owner) is within range of both A and C.
See Android's developer documentation for more info:
https://developer.android.com/guide/topics/connectivity/wifip2p