If you mean that you have 3 regular Pokemon caught, I think you should be ok, but I've only heard it from one person so I'm not going to promise it. It seems that the legendaries get generated before normal Pokemon, so the Tornadus should be in slot 1 for RNG purposes. Good luck.
Sorry for the late reply. I don't personally know how having other occupied slots effects the way in which Therians are generated as they have their own slot, So it is possible that the Therian could be generated before the other regular slots are. Maybe you could ask one of the original researchers how it works? Sorry I can't give a more definitive answer, but if you decide to proceed if an attempt, could you post your findings?