I. Love. This. Workout. It is not the most fun thing while you are doing it, but if you are wanting a great cardio session in under 20-minutes, you got it. 3-2-1 workouts have been around for a while and for good reason- they work. Runners who add 3-2-1 intervals into their repertoire of training find that it improves there run times. This has been true for me as well. Really, any interval training helps my newbie runner legs, but this one is my favorite :)
A 3-2-1 workout is ridiculously simple and can be set up with any smart phone app quite easily. If you are wanting free, Seconds Pro Free is a pretty great option. Seconds Pro is also great and saves workouts for a few bucks more. When setting it up, make ten sets of intervals of 30-seconds, 20-seconds, and 10-seconds, in that order. That's right, you will be running this interval ten times over. To vary it a bit and add to your workout, try doing ten 3-2-1, ten 1-2-3, and ten 2-3-1. Now, that is a 30-minutes workout that is worth the sweat effort!
When doing the 3-2-1, it is important to keep the walking, jogging, and sprinting times the same. That means that your run for 30-seconds, sprint for 20, and walk for 10. If you are new to running, try inverting this so you are walking for 30, running for 20, and sprinting for 10. It may not be as intense a workout, but it will be pretty intense, all the same!