If you want to know literally nothing about Season 3, don't read. That said, I'm not spoiling any plot points, just giving a minor introduction into the basic idea of the season. Also, if you haven't seen the first 2 seasons, I'd advise you to not read ahead.
Season 3 basically revolves around Frank as the President and how he comes to realize that, even though he's good at taking power from other people, he's not as adept at actually using that power effectively. It's a very interesting look into the more personal side of his life as well as his shortcomings. So far, we've only seen how aggressive and confident he can be, and season 3 starts to show us how that's not always a good thing.
Overall, it doesn't have the same feel as the first 2 seasons, because Frank isn't desperate to grab more power. He's at the top, there's nowhere to go. Things, IMO, seem to progress a bit more slowly and Frank realizes that he's not necessarily the puppet master.
End of spoilers