Yea.. I just can't even think of a plausible way you'd think there is any reason to even mention 'the '3' dragon riders' if you aren't familiar with the books. There's nothing in the show suggesting that anyone, anywhere, is going to become a dragon rider. Much less 3, specifically. I could see predicting Dany, and maybe even Tirion, learn how to ride the 2 dragons left behind or something along those lines.
I've heard about that prophecy from random internet sources, but the details are fuzzy to me as I haven't read through the books. I know it has something to do with Melisandre's comment about the 'prince that was promised' though.
I'm just hoping Jon becomes the Warden of the North when Dany finally invades and roflstomps Westeros, with Tirion as her Hand of the King(Queen). Littlefinger gets the throne briefly before she invades, just for fun.