I don't believe "Jon will get north tyrion south and dany for Essos." That's too cliche. SPOILER
I do believe they are the three heads of the dragon. But their fate after they defeat the White Walkers won't be very joyous. I think Dany will die in the end since there's foreshadowing for it. Tyrion will live but go blind (foreshadowing again) and Jon will be the unwilling ruler of a desolate and war-torn country. There's not going to be a happy
ending. GRRM said himself that the ending will be bittersweet.