Dynamic poses are always a pleasure to look at! You've done a great job at trying different angles, poses and involve objects, which are usually really hard but very important to do. In the first sketch you've also added depth by using shadows. I really like that. :D
Something specific I noticed is that the collarbones you draw feel a little bit glued on to the chest. They have a very specific purpose of helping your arms move around. They're attached to the chest in a certain way. Give this tutorial a look-over so you understand how it attaches: http://jilllenad.deviantart.com/art/Anatomy-Tutorial-Collar-Bone-217101968
In your second sketch, the belly button feels misaligned. I get the impression I'm looking at the body from the front, so the belly button should be in the center. It feels too far to the right though. The head and neck also feel placed too far left.
In the third sketch there are minor proportion issues. The head could be a bit larger, and the hand that's pointing away from her back is too large and could be sized down a bit. Her left (our right) breast isn't attached properly to the chest either, it's too far to the side and moving backwards too dramatically. Its attachment point is lower.
All in all, I really like these sketches. :) They incorporate many difficult elements but it's clear to see where you're going. If you pay attention to these pointers, you should be able to improve tons! Well done!