Hi!! Happy new year 
Did you think that TurtleBot3 can stand only two wheels?? Of course, it can’t stand.
But OpenCR and math makes it real!! The 6-DOF IMU (Acceleration and Gyro sensor) in OpenCR and PID control theory help the TurtleBot3.
The OpenCR is open project like TurtleBot3. If you have any interest of it then contact Github repository.
Also, If you want to study about PID control theory and apply the robot, then Buy This Cool Stuff after 3 months 