first off - nice going! I like that you do both life drawing and what to me looks like street life drawing (drawing people as they walk by) I try that often, and its insanely difficult! yours look quite neat though : )
I think you know, but the third drawing is (in my opinion) the best, this is because of multiple things: 1) you stuck with one medium, 2) the drawing is dynamic and balanced correctly and 3) it has mass.
To address these three points for the other drawings I would say that you need to stick with one color and one medium per drawing (but not per session, bring whatever medium you would like to explore : ) this will bring the focus back to the figure and its tonality, rather than to its colours. Then, try to make long fluid strokes (rather than short, precise ones) you did it in the third drawing, so you clearly know how : ) and last, be sure to make the down planes dark and the up planes light in order for the mass to read : )
I feel kinda silly telling you this, cause in the third drawing it looks like you know all of that(and frankly, that drawing is better than anything I ever draw in a croquis session)..so.... maybe its just the challenge of the paint? If you want to do paint, you could experiment with painting the planes rather than drawing the lines, : )
I don't mean to be harsh, I just think there is potential for even more goodness : D (sorry for rambling :P)