I really like the first one. The colors and play of texture is great. I think she needs to take a breath though and flatten her stomach. You could also be real creative and clean it up with Photoshop, but I would try enhancing the pose next time. The enlarged abdomen is a result of the beautifal back arch she has.
Number two has way too much shadow which is making her disappear. A soft gold or even a white reflector would dampen the shadows and give her more emphasis, and finally remove the contrast from her face. Great pose, maybe just a few more degrees of forward angle of her legs to add some shape to her glutes and add some angles to the composition
Not that number three is bad, but I would prefer to see her eyes--just my personal preference, the rest is great.
So, to answer your question about how to improve, get a reflector and work on details of the poses. Otherwise very, very nice work.