Ok, I managed to solve this pickle. I'll write it down, so others, who might have the same problem, can try it.
I found a software called ReclaiMe Free Raid Recovery. Its for Windows, and its free. After you tell it which drives were in the raid0, it scans around, depending on the size, for sever hours. For me it took about 2 hours until it was confident to have found the correct parameters for the raid0.
And so it did. It found them, and then it can either open up recovery software, or just tell you the parameters. With those, I've gone back to linux and used
mdadm --create -l 0 -n 3 -c 64 -metadata 0.9 /dev/md1 devices The crucial thing here was the metadata number. Apparently it defaults to 1.2, but my raid hat 0.9. So I basically tried them out and used fsck to see if it works. With 0.9 it found a filesystem which appeared to damaged.
Using e2fsck, it corrected a bunch of errors in inode 7. At one point, I cancelled because it seemed to do nothing. But after that I was able to mount it.
All files are there. I copied important stuff right away to another drive. All files I tested are working properly. The Raid has been restored.
Although there still seems to be something wrong in the filesystem, because
df -h shows that I am usung -27 GB :)
Thanks!