Hi there my friend,
I'll try to be constructive here, also I am not an expert it's just opinions of mine, but here it is:
There first one I wouldn't touch a thing. Great night time close up. Effective lighting that creates nice shadows and nice skin shade.
The second one is good, but I would make some tweaks. Her hair seems funky to me, her hair looks like it's catching wind around her neck but the hair stays on her shoulders. It's not a big thing but seems off a bit. Also I would try to position a light source behind her so her hair would cast a light shadow on her face. Then again it always depends on where your primary light source is coming from. Also seems like you are going for a beachfront sunset, if it is, I would decrease the temperature of the light source so it would be a bit more dark yellow/orange.
The third one, again great night scene with effective light/shadows. The only thing I would tweak here is the positioning of your model. It seems somehow off. For instance, there a weird bump/shape in her neck on her left. I'm sure I could position her head in roughly the same way without craning her neck so much. Her left arm under her boobs seems a bit off too. Her hand comes at an unnatural angle. Once again it all depends on how you try to position you model. It isn't always easy but I usually try to have my character in the most natural position possible. I would drop down her arm a bit and straighten her hand up. Also sometimes the position of the arms/shoulders makes clothes look funky. The straps come at slightly weird angles. If you play around with it, you might be able to straighten it up, but then again, you are restricted to the amount of modification options there is on the asset. Sometimes the asset have a crapload of modification options and cheaper one haven't got any. I can't tell you how often I settle on clothes just to find out that in certain positions it looks weird and can't make it look like I want. At that point I try to find another piece of similar clothing that might look like I want.
There you go, just my opinions, you actually do good work already, just need, like everyone of us some more practice. Play around with it, don't be afraid to fuck up and try new things, you might discover new ways of making your renders come alive!
Take care my friend and happy rendering!