slava @ rethinkdb here. We don't support Python 3 out of the box because rethink uses protocol buffers for communication between client drivers and the server, and the protobuf library hasn't been ported yet to Python 3. As soon as it does, we'll support it officially.

If you'd like to fund the porting effort of the protobuf library to Python 3, shoot me an email to [email protected] and we'll see if we can arrange something. Otherwise, I'm sorry, but you'll have to wait :)