Pretty nice stuff, First one is your best, the other two seem too bright and generic.
First: Nice composition for this one. It could easily do without the stalagmites though, and perhaps you could add some small grass doodads around the base of the boxes. Scale down the mushies too. My biggest problem is the fact that there is a moon in the back, yet it's light is coming from above the mine. Get rid of one of them, i'd say the moon, keep the light. Another major issue is that you could do some fog work. Like adding a backwards fog which is darker near the camera and gets lighter farther away. Use something like Z-end=0, and Z-start=4500, or whichever ends just before the end of the tunnel. Then add one or two semi-bright glow doodads outside. Another then you could try is use a glow doodad with a high z value, and very low x and y values for the Light Ray.
Second: It's a clusterfuck, learn to use your doodads in a coherent manner so that you don't have more than 3 different trees and no more than 4 different, repeated ground doodads. Also make sure that their colors compliment each other.
Third: Not bad, but like your second one it's very messy. Try and clean it up a bit and it should be golden.
As a good rule of thumb, you should try and configure your camera with a field of view around 100, or if you want a very large field of view do 120. This shouldn't be used above 80 if you're in a cave though. This can be achieved by creating a camera in the camera palette, and when you right click on the cameras name select "Edit Camera Properties. This'll make your terrains look more polished, just make sure you start the terrain this way.
