I agree that the first hand is already a little exaggerated. Is it Sargent? But also take a look at the wrist and its conjunction with the thumb. You have the wrist down lower. Your second finger middle joint is very high and that flattens that finger. The third finger is the one that should be flatter, but it's turned upward.
The second is a sculpture, and so is good reference. In fact, your thumb on the lower hand is darned good! The upper part of the index finger on the upper hand is a bit distorted, and I think the remedy is to do what Will calls "drawing through." He talks about how drawing close to the edge can tempt us to distort shapes and so we should extend outside the canvas to make sure our drawing is correct.
But mainly in this one, the wrist in the original has a lot more volume. I see where you've outlined the shadows, but I think you've got the whole thing a bit high. And also, I think that if your shading lines didn't just follow the form vertically, but described it by running across it more, you'd get more volume.
The third is the closest. Here you only have the ring finger coming out a little too far. But all of these tiny distortions are easier to see when your eye is fresh. I often draw something, go away for a while, and come back to see how distorted it is. Then I correct furiously for about five minutes.
Hope I'm not being too severe--and on your first post too! I'm a major hand stickler. And I agree with everything @Coreyartus said above. I don't think there's anything for it but concentrated practice! But you're off to a good start and practicing hands is so worth it, because they are so expressive! Continue to post your progress!