Here are a few things I see.
Image 1: As Scott mentioned below, the way she's posed and the way that the dress is spread out makes the model look pretty wide. Given the type of material in the dress, you'd have to be a REALLY SMALL model to NOT look wide in that pose. Also, the tilt of her head is a little off -- it may be tilted to portray "cute" or "carefree" but it just looks a little tilted.
Image 2 -- the first thing I noticed was the hat. But, it didn't LOOK like a hat. There's not enough of the hat in the image to clearly identify it at a glance, so it becomes a distraction. If you "take the tour" of everything that's in the frame when you shoot, you'll start noticing things like that. You have to make a decision to recompose or drop the hat.
Image 3 - I'm going to use image 3 to talk about the lighting overall. The color of the background and the color of the dress aren't different enough to cause separation and not complementary enough to fade together. You're in the no-man's-land between contrasting and complementary color. Particularly with that dress you probably need contrasting.
The lighting in general drops off on the right side of the image. Again, with that dress it promotes a more even light and more light. This is matching the mood to the mode -- the fashion needs to be integrated with the lighting. This is more a portrait lighting or boudoir lighting and you have a nice Sunday dress.
Image 3 also has that hot spot on the background that looks like a portrait effect or an attempt to separate the model from the background. Good thought -- just not enough light and the background's color isn't helping you.
Image 3 is really getting somewhere. Nice expression from the model. Nice sexiness with the Sunday dress used to expose and entice.
Bottom line -- two suggestions. Match your lighting (and background) to the mood of the piece. Take a careful "tour" of the viewfinder whenever you compose and see what you've got. It's important to make sure the eyes are in focus, etc, but the money comes in the rest of the details!