Marianne, The problem isn't really related to the fact that the filesystem is spread across 3 disks. It just happened that the internal structure of the FS used 64 bit group descriptors.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHBA-2016:0616