Three years on from “Game of Thrones,” author George RR Martin and the cast of new prequel “House of the Dragon” discussed the daunting challenge of following up HBO’s wildly popular series at Comic-Con on Saturday. Martin’s series of fantasy books spawned “Thrones,” as well as several upcoming spin-offs. The first — out August 21 — will tell the history of the murderous reigning Targaryen family and their 17 dragons. “It’s pretty exciting. You’re always nervous at the beginning, because these books, these characters are like my kids,” said Martin. “You wonder how they will be treated, will …