A tricopter uses 3 motors and propeller propulsion units with servo which rotate one of the propellers to compensate for the adverse yaw.
In the beginning … here you can read about how it all started. A nimble tricopter, equipped with stress balls to soften a hard landing, quickly evolved into a more sophisticated and much larger drone, which also meant moving from KK2 to ArduCopter/APM 2.6 ArduPilot to 3DR Pixhawk.
THE ELUSIVE BLACK PHOENIX
Here are a few more photos of the tricopters and a short video.