Question: how I can make my collages more 3D or less flat?!

Think realistically. Use shadows, perspectives, scale, layers. Texture can also do wonders. You bring to the table what you know and build on it. Those programs have some wonderful tools to help. Don't be afraid to scrap stuff you work hours/days on. It's good practice. Also- abuse the tools/addins... sometimes the results are interesting and can give your work its own special quality. By DFT.

Shadows, perspectives and scaling are essential. Depending what you're trying to create, keep light sources in mind. You might also want to extract people/images from their backgrounds without feathering, and experiment with lighting effects and shadowing. 

Remember that shadows aren't feathered black silhouettes displaced several pixels up or down and to the side of what's casting them. That is, if they're 3D, or faux 3D. They're warped, blurrier in certain parts, more opaque in certain parts, and aren't black. Look around your desk at the shadows things cast and adjust the light source in the room to see how they change. Also remember to give certain objects (plastics, metals, surfaces, etc) highlights in your images (airbrush is good to use). By emtarte.