D 3 number 12 wires equal one number 8 wire?
This is not code at all.
Unless this is in conduit, the following are the reasons.
All your conductors need to be bundled together (as in romex or UF cable)
And your Neutral is not allowed to be undersized.
Get us a picture if you can because this sounds like someone messed it up bad.
Also, if you have a subpanel you also should have a Ground Bar that is wired to the main's neutral/ground.
"Do it right the first time and avoid duplication of effort"
Your AHJ/Inspectors ALWAYS have the final say on ANY electrical code issue.
If in doubt, contact a licensed, experienced, reputable electrician to perform the work.