There are 3 Houses. There weas only 1 line when we observed the 1st house, then 2 lines inside the O when introduced to the 2nd house. Finally, 3 lines inside the O when the 3rd hpuse was introduced which remained through season 6.
https://www.quora.com/Why-there-are-three-lines-between-Game-of-Thrones-logo
...three branches of government?