The third button on Android is the multi-task button. The back button allows you to go back to the history of activities (task you can perform in an app) you've opened before arriving at the current one, but if you want to specifically jump to a certain app you already opened, the multi-task button is faster. If you want to open a new app you can switch to the home screen with the home button and open it from there (or from your app drawer). This leaves the activity history of the app you had open intact, so when you open the app again it resumes where you left.