You can already now harvest the foldername to get the discnumber.
See the FAQs: Converters to rename files or to import tags - #3 by Florian
If you sort this set of files by track number, so that all the 1s stick together, then the 2s and so on, then you could assign a discnumber with an action
Format value for DISCNUMBER
Format string: $add($div(%_counter%,3),1)
This should set 1 to the 1st, 2 to the 2nd, 3 to the 3rd file and then 1 to the 4th and so on.