DenisG and others,
It seems from the posted info that Airbus is using its three ADIR units to provide three separate IRS positions. Probably these are then compared with a "dead-reckoned" new position obtained from time and speed. These two pairs of the three positions are then apparently used with Kalman filtering to obtain a new adjusted position. I don't know how three possibly different corrected position values are handled. I assume than one or more of these were in error due to the errors in the speed measurements, which may have grown slowly over time. In my personal opinion, this may well explain why the transmitted FM-position perhaps was more and more off from the correct position near the end of the flight.
Any comments?