Tri-copter

what is Tricopter?
A tri-copter is a multirotor aircraft that uses three motors and propellers
 (arranged in a horizontal triangle) to stay airborne. Tri-copter is unlike
a helicopter that rises on a single column of air, the tri-copter rises
on three columns of air making it more stable. It uses 3 standard brushless
out-runner motors.

Direction of rotors rotations 
 The three rotors are divided into two right-handed and one left-handed,  
one-right-handed and two left-handed, or three right-handed.
In order to balance the yaw torque produced by the three rotors,
the RC servomotor and linkage structure is installed on the tail axis,
in order to improve the angle of the rolling axis of the motor on the tail axis.
 Moreover, through the torque generated by the horizontal component of
the lift from the inclined motor (motor no.3) on the tail axis,
 it balances the yaw torque of the three rotors.

Tricopter Theory of flight

Forward Flight: when flying forward, motor 1 and 2 must decelerate,
 while motor 3 on the tail axis must accelerate. As a result,
 the fuselage of the Tricopter is inclined forwards, so it flies towards the same direction.
Backward Flight: when flying backwards, motor 1 and 2 must accelerate,
 while motor 3 must decelerate.
Fly to the right direction: when the Tricopter flying robot flies to
the right direction, motor 1 on the left side must accelerate, while
motor 2 on the right side must decelerate, so as to allow the
fuselage to incline to the right side and make the Tricopter fly to
the right direction.
Clockwise yaw: when the Tricopter flying robot yaws in the
clockwise direction, it needs to use the RC servomotor and
linkage to drive the propeller of the motor 3 inclined in the left
side. When the motor 3 rolls, it will generate the clockwise yaw
torque, so as to make the Tricopter flying robot yaw in the
clockwise direction. 

Advantages of a Tri-copter:
1-three points in space define a plane: The Tri-copter is very agile
2-yaw Servo makes yawing faster so more maneuverability
3-lower than quadcopter in power consumption   
Disadvantages of a Tri-copter: 
If you lose a motor you will crash instantly...no redundancy
yaw mechanism and servo motor problems
more rotors means more stability