Write notes in a notebook about people you see. Keep track of their actions, the way they speak and their body language. Practice writing physical descriptions. Do this often and keep the notebook handy to refer to when you develop characters for your stories. Learning how to observe and capture what you observe is one of the best ways to develop characters with three dimensions.
Develop character sketches for each character you create. Mix and match the information in your notebook, using various features to create characters with depth. Include their physical appearance, character traits and their emotional makeup. Building these dimensions into your characters will help bring them to life. Don't be afraid to include information in your character sketch you may not use in a story. The more you know about your character, the better you will be able to breathe life into him when you introduce him into a story.
Write your story with your character notes handy. Keep the traits of your characters in mind as your write. Since this is a character-based story, select characters suited to the story and use the story line to enhance their personalities.
Compare your characters as you go along. If you can interchange them throughout the story, you have not fully developed three-dimensional characters and you'll need to refer to your notebook sketches to find the information most essential to giving your characters distinct 3D personalities.