Hi SW,
Some easy Explanation would help
The ISIS is stand alone Artificial horizon, Speed and Altitude (from own gyro, standby probe and static vents).
The input from ADIRS 1 and 3 are for heading information and something called "Accelerometer Gyrometer" (whatever that means).
https://www.livescience.com/40103-ac...gyroscope.html explains why it is better to have a mixture of gyroscope (from ISIS) and acceleration information (from ADIR) to give a better attitude reference.
It was discussed at length here:
A320 ISIS. What are the ADIRU inputs for?