We wouldn't advise anyone to go out and try to run 3 miles when they are starting a new journey toward a healthier lifestyle. You will want to follow this journey for the rest of your life for permanent and long term success so you have to learn new skills and routines as you go. If you desire to become a runner, here is an article that can help you build up in walking or jogging.
Spark Your Way to a 5K
5K Training Programs www.sparkpeople.com/resource/fitness_artic