[Ken Bennett] “BUT, I did just noticed #1 and #3 are formatted as Windows NT Filesystem, whereas #2 is Mac OS Extended (Journaled).”
I believe you have found the problem. No version of macOS could write to NTFS drives without additional software (read but not write). That would explain why it is reporting that the drive is full. I’m guessing that whatever you used in Sierra to write to NTFS volumes has stopped working in High Sierra.
I just plugged my NTFS formatted RAID5 into my High Sierra MacBook Pro and I’m seeing the same thing.

[Ken Bennett] “I’m guessing this is the problem and I should move data of #1 and #3 and reformat to Mac OS Extended (Journal)???”
Yea, if those are old drives left over from your PC days it’s time to reformat them.
~jr
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