Three Body Problem is not even remotely close to hard sci-fi. The whole notion of "sophons" is ridiculous, for example, and the more the author tries to explain

Three Body Problem is not even remotely close to hard sci-fi. The whole notion of "sophons" is ridiculous, for example, and the more the author tries to explain how it works, the more nonsense it ends up being. If you want another example of actual hard sci-fi, I would suggest Lem's Tales of Pirx the Pilot. To give an example, I remember it being pretty much the first sci fi book that not only pointed out that laser beams are invisible in vacuum, but actually made it a major plot point of one of the stories.