All three twice. I like to really understand what's in there so when I feel I need to make a house rule I have a proper perspective on it, so I can be more consistent in my DMing, and so I'm pretty darn sure what is and isn't in the books (details will still need to be referenced).
I read all the way through the SCAG too, though I didn't pay close attention to some of the setting information that wouldn't have been accurate in 1371 DR. If an RPG book isn't enjoyable enough to want to read all the way through (sure, some parts are always a slog when read that way), then I probably regret my purchase.