It would seem that a disk (/dev/sdc) has failed, and Cassandra's data directories for all 3 instances are unwritable. Since these instances have already been down for some time, and no ETA for repair/replacement yet exists, the safest choice would be to remove the instances.
- [operations] Shut down node and replace sdc
- [operations] Re-assemble / re-create RAID0
- [services] re-bootstrap cassandra instances