It is clearly unknown if there will be a third season but it is somewhat hinted in this article. Overall it would be up to the creators to decide if they wish to make a third season or not but the possibility can be moderately low to high because there is so much we do not know about geass and its existance. It is clearly obvious C.C and V.V or charles zi britannia are not the only code givers in the series [or maybe L.L]