If you are using SATA, the disks may be in a different order when the computer boots up. SATA disks in Linux is usually are SCSI devices, so it is best to set their ID to make each disk comes up in a predictable order. I think some hard drive manufacture utility can set the ID. If the disks are IDE, then you do not have an ID problem but problem in hardware such as a disk failure, cable disconnect, or controller failure.
I suggest read the logs if you can because they may give you clues. If the order of the disks is messed up, edit /etc/mdadm.conf to put them in the correct order. Then try mounting it as read-only. If it does not mount, you may have to use fsck to fix the problem. Hopefully, you can get it up, but RAID-0 with three disks is very, very dangerous.
I have not mess around with software RAID, so this is as far as I can go.
FYI, its directory not folder.