Obviously there will be a 3rd dragon rider, John obviously is #2.
You could be right about who #3 is...
I was watching the last episode again and in the scene where Eddard Stark was with Lyanna Stark something occurred to me. Both Tyrion and Jon Snow killed their mother at childbirth. Could this be something that happens if a Targaryen child is birthed by a non-Targaryen mother? Having babies outside of the Targaryen family may explain the dark hair.
Just wondering.