Pretty nice stab

. My first suggestion for improvement would be to shoot raw - always, but then you just knew that was coming, didn't you

. It gives you many more degrees of freedom in your post processing, regardless of what else you may be trying to do.
I can't comment on the Nikon gear (I shoot with Canon), but the results seem to suggest that the combo is a good start for landscape. It would be nice to know what the technical aspects of each image was - exposure, aperture, speed, etc., but that may just be me as I am a little more oriented to the techie stuff than some. I ask, because it is often a bit difficult to provide "good" feedback without knowing the details of how an image was captured.
Having said that, my comments are:
1. I like this image from a compositional perspective. For me it is a little underexposed maybe by as much as one stop. You may be able to correct this in post, but then you run the risk of blowing out the water, so you may want to do some sort of selective dodging and burning to bring out the colours of the trees and other vegetation while not blowing out the waterfall.
2. Comments are much the same as for #1. The vegetation in the foreground covers up the "boiling" water of the falls and I am not sure that is something I would have done. The exposure of the vegetation in this one is much better than in #1, but could probably use a bit more "brightening up" as the colours here have great potential. Don't forget, that even though your main subject may be the waterfalls, the surrounding "countryside" is also an important component of the image. I would have also cloned out the picnic bench on the right middle side of the image. It is a distraction and grabs the viewer's eye away from the main subject.
3. I like this one - a lot. The colours are great although they could be a bit "brighter". The S-curve of the pathway carries the viewer's eye through the image very nicely. The bench provides a very nice touch and is a good place for the viewer's eye to pause and explore the image as they follow the path through the image.
Hope this helps.
WesternGuy