It looks like you added a gradient layer or a vignetting layer which has caused the first two to look underexposed. If you do add these layers, it is best to boost the brightness of they underlying layers so they don't come out underexposed. You could also increase the transparency of the gradient layer to reduce the amount of vignetting you have going on. I like the effect when it's done right, but it doesn't really contribute anything to these photos.
Sam's advice about the angles you shoot at and composition would also help you improve these photos. Nice work otherwise!