I feel your frustration, I am new to Nowtv and I was watching the same show and saw last night that it had an hour left for each episode so I downloaded all the episodes on my android device. Now season 3 has disappeared and the episodes are in my download by I cannot cast to my telly. Customer have told me that downloads cannot be cast so they are forcing me me to watch on my mobile. This is not a good look Nowtv, if any customer care is reading, this is really bad value for money
I don't get this, how come I can still watch seasons 1 and 2 and they have been out for years, yet nothing from season 3 ?
The answer is in your question; seasons 1 and 2 have been around for a good while, so the rights are cheap and easily available, and Sky/NowTV have reacquired them accordingly.
But with season 3, Sky/NowTV got the sought-after first run rights, which only give you 30 days catchup per episode, after which HBO can sell streaming rights to anyone else for a good price.
Expect season 3 to reappear in a year or so, when HBO have pretty much sucked all the juice out of it.