Just to give one example, at one point the first book features an integrated circuit somehow "etched" on the surface of a stretched-out elementary particle. And that's not just a random curiosity, but a key plot device, without which said plot collapses.
I can't in good conscience call this hard sci-fi (which is how the book is usually advertised).
If the Trisolarians had sent them out in various directions to scout out solar systems they could have avoided so many headaches.
All of these components are lacking in the book, and it's not just the particles. It reads very much like someone subscribed to some popular science journal for a few years, and then just dumped every idea that sounded interesting into a book without much understanding of what it is about.
To give another example, the author references string theory - great! - but then he has a one-dimensional string - that makes up an elementary particle - break apart into "pieces" from gravity.
Or take alien propulsion. It supposedly works by collecting antimatter from space - but where in space is there enough antimatter to collect like that, and why hasn't it annihilated from interaction with the much more prevalent hydrogen? The interesting thing about this example is that it's not insurmountable - hard sci-fi could explain it away in some convincingly sounding way - but the book doesn't even bother to, it's just presented as fact, without even pointing out the contradictions.
Do we have any scientific evidence that any of higher-dimensional antics in 3BP are possible?
If not, then the author is making up his own world building rules (completely acceptable). But that makes the story more akin to scientific fantasy than hard scientific fiction.
And unless the English translation excised a LOT of technical details, there are some huge leaps of scientific faith in 3BP.
Even that much is being overly kind (at least to the first book - I gave up after that). Hard SF introduces a small number of well-defined ideas and then works through their logical consequences. Science fantasy introduces magical devices with particular consequences as and when the plot demands. The magical VR system and magical particle computer - essentially all the SF elements of Three Body Problem - were examples of the latter.
If you're going to take 1,800 pages (English translation) to lay out your series, and you can't spare a few for 'well, now we're building giant fusion-powered warship fleets'? That's not anything approaching hard sci-fi in my book.
Yes, all the technologies used on the ships are extrapolated and believable, but the realignment of society, economics, and the organization of such a construction effort boggles the mind. And to say 'it just happens'? That's kind of like going from WWII to 'and so we landed on the moon' in a single page.
On the one hand, one obviously needs some technological leeway to write a fun novel set in the future.
On the other, if every roadblock is responded to by suspending scientific treatment, it doesn't seem like that's a good faith "hard" effort.
I think Ringworld is instructive. The Wikipedia article specifically calls out technical inaccuracies in the original... inaccuracies Niven specifically made an effort to patch up in later books.
So the count of intentional lapses seems as decent a proxy for hardness as anything else.
Da Shi (together with Luo Ji, my favourite characters), Ye Wenjie, Yun Tianming and Thomas Wade develop across the series. If you cast the various human masses, the Trisolarans and Sophon as characters, which they resemble, they—too—develop compellingly over the series. Certain family lineages also cast favourably as “characters”.
It does take a different view of character development to make things pop. For me, the best part about the characters was experiencing Liu’s worlds through their odd perspectives. (Cheng Xin was not a pleasant perspective. But against Wade’s and Beihai’s foils, it is thought provoking.)
I'm on the "don't care about character development" side. But those books are atrociously slow. The whole "3 body problem" could be condensed to maybe 50 pages.