[ras'matroi]

I still think that Tyrion could be one of the heads. I think it has been discossed before but in his second pov in AGoT it says:

1. He reads some rare books on dragons, he borrowed from the library of Winterfell.
2. 'Tyrion had a morbid fascination with dragons.'
3.he looked for the dragonskulls, when ha came to King's Landing, found them and thought them rather beautiful than frightening
$. He tells Jon how he dreamed of riding a dragon as a child and that ha was fascinated by fire.

I don't remember if there were other passages like that. But to tell this to the reader at the beginning of the story, when the characters are still introduced it appeares to be important.

I don't think that Bran will be one of the three but probably he even so will be able to controll one of the dragons. His power is still growing.

If Jon really is part Targ it might enabel him to ride and controll one of the dragons. The same as the Szak blood enables the Stark kids to bind with the direwolves. This theory would mean that there has to be a third Targ. That's if only Targs are able to team up with dragons.