what are the locations of the cube orange+'s IMU sensors. I can’t mount it on the center of vehicle. But I need millimeter precision. Could you inform me about this ?
Usually that level of precision is not reguired. It helps a lot if you can run the calibrations with the flight controller fixed on the craft in it’s permenant position.
Just estimate the IMU positions at the centre of the Cube, and you can set these params to suit each IMU in Ardupilot
INS_POS1_X,0
INS_POS1_Y,0
INS_POS1_Z,0
INS_POS2_X,0
INS_POS2_Y,0
INS_POS2_Z,0
INS_POS3_X,0
INS_POS3_Y,0
INS_POS3_Z,0
The value is in meters ! so be aware 
It was probably put in place for very large craft where the IMU or flight controller is a long way from COG.
Look up the parameters in
https://ardupilot.org/copter/docs/parameters.html#ins-pos1-x-imu-accelerometer-x-position
thnks a lot