Junit 4 and Selenium – Part Three: Parallelism and OnDemand` is a guide on how to run Selenium tests in parallel using JUnit 4. The author uses a custom `Parallelized` class to achieve this, which dynamically adjusts the thread pool size based on the number of test parameters. This allows for parallel execution of each test method regardless of the number of browsers or operating systems being tested. The guide also discusses how to use Sauce OnDemand, a cloud-based testing platform that integrates with Selenium scripts, to run tests in parallel without requiring the user to have multiple browsers installed locally. To accommodate this, the author uses a JSON string to pass configuration information to the Sauce OnDemand server, which is built using the Gson library. The guide concludes by highlighting the benefits of running Selenium tests in parallel and providing resources for further learning.