Part Three of the tutorial series focuses on building a real-time map application using the Google Maps platform and PubNub for geolocation tracking. The tutorial explains how to use the HTML5 geolocation API to collect the user's device location and update Google Maps JavaScript API map markers in real-time. It covers the setup of JavaScript environment and map markers, using the HTML5 location API for real-time updates, and implementing PubNub for data streaming. Key steps include defining map and marker variables, initializing the Google Maps API, handling redraw events for position changes, and using a JavaScript interval timer to publish updated location data. The tutorial also highlights the importance of obtaining an API key from the Google Cloud Platform for the demo and sets the stage for the final part, which will introduce flight paths to track device routes.