I just used blackhornet's converter on the c2 plugin and it seems to work now in c3. I don't want to host the converted files, because I can't help debug problems and have not tested more than just a few functions like authentication, responses, messages and a custom logevent, but it worked with my gamesparks preview account and my custom event.
Pointers to files:
C2 SDK for Gamesparks (get the plugin dir)
Converter thread with tool link and instructions
I found it very useful to debug gamesparks requests using the chrome console, so you can see all the requests go across between chrome and gamesparks server. (Ctrl-Shift-J for Chrome when game is running in Chrome.)
blackhornet, thanks again for the nice tool, I guess it's feeling a little dangerous (at least just for me) to convert others addons and host them, since they are not 'official' conversions, but I have no problem showing how folks can do it on their own, if the 'official' owners have not converted yet.
As an aside with the gamesparks *.capx demo, I found that the request/responses were causing the game to stutter, which was interesting, something to watch for in the future.