Doesn't Euron have the horn? I can't remember from the books. I know Victarion is on his way there.
Anyway, there are three dragons so I think the prophecy sort of fits when there are in fact 3 dragons. As for the horn it allows you to gain more control over them, but as we've seen from Prince Martell, not everyone can touch them. So only a special few people can in fact ride them. Obviously we have seen Dany ride them. Most assume Jon is Dany's nephew and with his ability to warg would make perfect sense or maybe he isn't, who knows. If those are the two I have no idea who the third would be. Maybe it is Bran, maybe it is a Greyjoy, maybe the guy who claims to be Dany's other nephew is actually her nephew and it is him, maybe it is Tyrion, or maybe there aren't three riders and it is false prophecy. Who knows. Should be interesting.