I think there will be three, and that Tyrion will still be one. There are theories that you don't have to be a Targ to be a dragon rider. In the past (novellas and stories) there were a call for Targ bastards to try to ride dragons, and a number turned up, though there was no proof they all actually had Targ blood.
Yeah, this created a bit of a problem. They either broke with the mythos they already created (you can only control one dragon), which is historical in Westeros, or they made it variable so there don't need to be more dragon riders. Either way, it doesn't seem to jive, but whatever.