I have not read the third one yet. The main thing that people find "weird", I guess is actual character development. One could argue it has little to no bearing on the story, but I would argue that if you only read for story/plot, you aren't really reading.
With my little soapbox speech out of the way, the first two books do offer a pretty complete story. I had to go straight to the Dark Forest after Three Body Problem, but have been able to take a year break for Death's End, not because I didn't like it, but at least as far as I can tell, a lot of the major plot points get resolved pretty satisfactorily by the end of Dark Forest. Enough that I can get lost in other books!