In-App Purchases using Google's in app billing version 3. (google-iap-v3) > config.lua > Key

I’m not sure which key to provide for config.lua (see below).

Is it the Licence key meaning the  Base64-encoded RSA public key?

http://docs.coronalabs.com/daily/plugin/google-iap-v3/#config.lua

application = { license = { google = { key = "Your key here", }, }, }

Yes.

Yes.