The number 3 as you have pointed out has many predesessors for references. That I can accept. Many authors, including GRRM use these age-old concepts and weave them into their writings. The thing about Rendezvous with Rama is that in the very last paragraph, the researchers realize that the Ramans do everything in 3's. Their first space probe is leaving our solar system, but they realize that they can expect 2 more to come in the future. If we apply logic to ASOIAF, then we can speculate/predict some possibilities that have been hinted at in ASOIAF. If things happen in 3's, then it is most probable that: 1. Rhaegar had a 3rd child....probably with Lyanna....and it is probably Jon. 2. Jon switched 1 baby (Mance's) to save its life. Perhaps 2 others have been/or will be switched. One theory is that Rhaegar's son, Aegon was switched and is still alive. Another theory, though far more crackpot, is that Mirri Maz Duur switched Dany's baby and left a dragon fetus in its place. (I think that was Nightflyer, but really, when one thinks about it, it is hard for me to believe that Dany went into the tent with a very alive and kicking human baby and gave birth to a dead dragon fetus). But if not this baby, then another one somewhere in the series could pop up. This isnt a post to debate the specific speculations raised above, but to point out a theme in the books. Look for things/events/concepts in ASOIAF to happen 3 times.