Hi Marco,
The tricopter is in priciple the same as the bicopter, you just need to change your motor/servo mixing.
Traditionally, the tricopter has 3 motors with the rear motor on a servo operated platform to control the aircraft's yaw. The arms are configured to form a "Y" and are usually spaced 120 degrees from one another, or thereabouts.
The motor mixing for a tricopter is as follows:
Motor 1 (front left): Throttle 100%, Airelon: 87%, Elevator 50%, Rudder 0%
Motor 2 (front right): Throttle 100%, Airelon -87%, Elevator 50%, Rudder -1%
Motor 3 (rear): Thottle 100%, Airelon 0%, Elevator -100%, Rudder 0%
Servo (rear): Throttle, 0%, Airelon 0%, Elevator 0%, Rudder 100%, Offset 50%
The Arduino Nano however is not the best option to obtain high resolution PWM on all four outputs (3 motor + servo), as it only has two 16-bit timer outputs. A better option would be the Arduino Micro, as this has four, 16-bit outputs: three channels on timer 1 for the motors (at 490Hz) and one on timer 3 for the servo (at 50Hz).
On the Arduino Micro though, you have to be quite careful about pin selection if you're using an RC receiver, as unlike the Arduino Nano not all pins are capable of receiving interrupts.