I have three 1TB HDD (shows 931.5GB usable) running in RAID0 (shows RAID0 working and recognizing three drives), however, it looks like it's only recognizing two drives, as df -h is showing 1.8TB.
If the drives are working correctly, I presume they are or you'dbe asking a different question, then this is a linux issue.
I will move this post to our Software/OpenSource forum.
It will probably be helpful if you listed the steps & commands you used to create the array, as well as the "$ cat proc/mdstat" output.