In broad terms, the action in A GAME OF THRONES and its sequels is definitely informed by the War of the Roses, one of my favorite historical periods. It's not a one-to-one correspondence, however; I had considerable fun playing with expectations and mixing things up, and the characters grew more from my own head than from history.
Yes, the series was originally a trilogy, but it has grown... to four initially, but now I am inclined to think it will be longer than that. What can I say? It's a BIG story, and a cast of thousands.